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acid, will be nothing more than a solution of pure acetic acid LESSONS IN CHEMISTRY.-No. XXIX.

(vinegar) in pure water.
Two very important precipitating agents for lead out of its

Fig. 26.
solution yet remain to be discussed: they are oxalic acid or
its soluble combinations, and sulphurous acid either free or
combined,

The completeness with which oxalic acid, or, still better
perhaps, oxalate of ammonia, precipitates lead from its solu-
tions admits of ready demonstration. As respects the case
of acetate of lead, let the pupil add oxalic acid in solution
or oxalate of ammonia, until no further precipitate is deter-
mined. This condition having been arrived at, let the solution
be filtered and the colourless filtrate be tested with hydro-
sulphuric acid solution. Under these circumstances, not the
slightest indication of blackness will be visible, a fact which
proves that the oxalic acid, or oxalate of ammonia, are complete
precipitants of lead out of its acetic acid combination.

Proceeding with our examination of oxalate of lead, it admits
of demonstration that the substance is exceedingly insoluble
in water, even under the operation of boiling. In point of fact,
oxalic acid and the soluble oxalates are remarkably efficient
and delicate tests for lead in solution.

Extraction of Melallic Lead from Oxalate of Lead. If oxalate
of lead be mixed with carbonate of soda and powdered charcoal
(black Aux is perhaps better) and heated on a piece of char.
coal with the blowpipe-flame, metallic lead results. It is
impossible, however, to conduct the operation in such a
manner that a portion of lead oxide shall not also be developed,
so great is the tendency of metallic lead to combine with
oxygen when heated to a sufficient extent in contact with Before tracing the application of sulphurous acid gas to the
atmospheric air. Oxalate of lead is easily reducible when sugar refining process, it will be necessary to examine the
treated as described, but it may also be stated that all lead effect of the acetates of lead generally (there are more than
precipitates under similar treatment afford parallel results with one; we have been dealing with the mono or neutral acetate)
various amounts of difficulty. Perhaps the sulphate of lead on certain organic solutions. For this purpose we will take a
is more difficult of reduction than any other lead compound.

solution of muscovado or common yellow sugar, about the Sulphurous Acid as a Precipitant for Lead. The suffocating gas strength of one of sugar to six of water (both by weight). generated when brimstone or sulphur is burned in atmospheric Solution having been effected by means of heat and agitation, air or oxygen, and termed sulphurous acid gas, is a complete add to it portions of solution of acetate of lead, or, still better, precipitant for lead out of its solutions, and is extensively used the tris-acetate, * commonly known in druggists' shops as for that purpose in the operation of refining sugar by sub-Goulard's extract, until most of the colouring matter has become acetate of lead.

deposited. I say most, because the latter portions of colouring For the purpose of illustrating this action, prepare a solution matter cling to the sugar solution with great obstinacy, and of acetate of lead, strength immaterial, and lighting, imme- cannot be gotten rid of without the expenditure of more scetate diately above the surface of this solution contained in a test- of lead than is from motives of economy desirable. You will

glasa, a brimstone match, blow the gaseous result of combus- observe that on now filtering the sugar solution thus treated, tion (sulphurous acid gas) down upon the surface of the acetate the filtrate will be almost devoid of colour, hence the acetate, solution. These directions being followed, a copious white or rather the tris-acetate of lead would be the very best subprecipitate will immediately appear; the white substance stance, could it be safely used, to be employed for the purificabeing sulphate of lead. The plan we have followed for render- tion of yellow sugar. It is quite evident, however, that the ing evident the effect of sulphurous acid on solutions of lead, employment of this material must remain out of the question is necessarily different from the plan followed in practice until the discovery of 'some inexpensive, readily-applied and whether in the laboratory or the larger scale of commercial unfailing plan of extracting superabundant lead" has been manufacture. The sulphurous acid requires not merely to be devised. All the ordinary precipitants for lead had been tried brought into contact with the surface of a liquid, but to be without avail, until Dr. Scoffern bethought himself of sulphurpassed through it in a stream. On the large manufacturing ous acid, which answers, completely. The process has now scale, the sulphurous acid generated by the combustion of been in extensive operation for more than six years, and no sulphur in atmospheric air is forced through

the liquid to be untoward accident has occurred from the use of the poisonous deprived of lead by means of a pump. In the laboratory, agent. pressure can be more readily obtained by generating the acid This is a convenient opportunity for the introduction of in a flask, a process which no longer admits of burning sulphur some remarks on chemical nomenclature, having special as the source of sulphurous acid. We must employ for this reference to the compounds of oxide of lead with acetic acid, purpose oil of vitriol, and generate our gaseous result by heat- and of very general application in other cases. You will ing that liquid in contact with charcoal, or mercury or copper; remember that I denominated the Goulard's

extract employed in general terms charcoal is the most convenient.

in our sugar refining experiment by the name of tris-acetate or Into a Florence flask, pour about a table-spoonful of wood sub-acetate of lead ; let us therefore investigate the meaning charcoal broken into small pieces, but not powdered, to this of those terms. Tris is evidently a prefix

signifying three of add about a table-spoonfull of oil of vitriol,

adapt a cork and something;

sub is a prefis which means an under or diminished bent tube as represented in the accompanying diagram, fig. 26, quantity of something. We will proceed to i: vestigate the dip the free end of the tube to the bottom of a test-glass con- exact meaning of these terms. Beginning with sugar of lead, taining acetate of lead solution, and apply heat.

this is the neutral acetate; it is composed of one equivalent of These

conditions being complied with, copious volumes of base (oxide of lead) combined with one equivalent of acetic two gases, sulphurous acid and carbonic acid, are evolved; acid. Now the chemico-algebraical notation or symbol for Permeate the liquor and throw down sulphate of lead. As to lead is Pb (contraction for “ Plumbum," and the symbol for the carbonic acid it does no harm, for so long as sulphurous oxygen is e, consequently oxide of lead (protoxide)

being acid passes, no carbonate of lead' is found. By proceeding composed of one equivalent of lead plus one of oxygen, its thus, the operator will discover that every trace of lead is symbol is PbO. removed from the solution, which, if exposed for some days to a warm atmosphere, in order to get rid of lingering sulphurous

• Otherwiso called sub-acetate, or Goulard's extract, VOL. V.

108

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Again, Ā is the contracted • symbol for acetic acid; there

And this is man:-Oh! ask of him, fore PbO +Ā or Pb O, Ā represents neutral or mono-acetate

The gifted and forgiven,of lead. But it so happens ihat there are other acetates of

While o'er his vision, drear and dim, lead, in number somewhat doubtful, perhaps five or six, all of

The wrecks of time are driven ; which contain an excess of base (oxide of lead, or PbO).

If pride or passion in their power, Indefinitely therefore they may be generalized as sub-acetates

Can chain the time, or charm the hour, of lead; but if we desire to be precise, and indicate what kind

Or stand in place of heaven: of sub-acetate of lead any particular one may be, we must have

He bends the brow, he bows the knee, recourse to numeral prefixes, and chemists have long agreed

“Creator, Father! none but thee !" to indicate all excess of base in any salt by prefixes of Greek origin. Thus mono-acetate of lead remains neutral acetate ; di-acetate, the particular subsalt generated by combination of two of base with one of acid; tris-acetate, the particular sub- ON PHYSICS, OR NATURAL PHILOSOPHY, salt generated by combination of three of base with one of acid,

No. XXX. and so on for the rest. Furthermore, it should be remarked, that, in addition to the general or indefinite word sub-salt,

CALORI C. there is another very much employed in modern chemical

Continued from page 36.) treatises : I mean basic salt; that is to say, a salt in which the base predominates.

The following table will be found of great utility in convertIn our next lesson we shall enter upon the important process ing the number of degrees on any of the ihree different scales of cupellation, or separation of lead from silver, and indeed into the corresponding number of degrees on either of the other gold by aid of fire. It is an operation of great beauty and of scales, viz. Fahrenheit's, Reaumur's, and the Centigrade. particular interest just at this time, when so many persons are This table will be extremely useful to Students of Chemistry devoting their attention to the study of the precious metals. and Natural Philosophy, especially in reading the accounts of

The operation of cupellation will require the possession of French and other continental discoveries in these sciences,
some bone ashes. If the student desire to enter very far into We shall have frequent occasion to refer to it ourselves, and
the practice of the art, he will also require several pieces of to make our Students refer to it in future Lessons. Experi-
apparatus, to be described in our next. The mere theory of mental Chemists will do well to purchase this number of the
the operation, however, admits of being taught by the aid of P.E., were it for the table only, as it will save their making
a blowpipe operation. The preparation of bone ash is a matter calculations, and prevent their committing mistakes in con-
of no difficulty. Into a clear fire put pieces of bone, and there verting the degrees from one scale into another,
let them remain until every thing capable of being burned In explanation of the following table, we observe that it is
away has been removed-in other words, until they become divided into three columns, headed Degrees of Fahrenheit,
quite white. Remove them then from the fire, and when cola Degrees of the Centigrade, and Degrees of Reaumur, respectively
powder them.

The table commences with the number of the degrees marked
at the boiling point on each scale, and gradually decreases by
I degree, and in some cases by parts of a degree, to -42° -25

Fahrenheit, - 41.25 Centigrade, and --33• Reaumur. Thus,
M A N.

suppose that when the temperature of a body is 2009 Fah.

renheit, and you wish to know what it is on the Centigrade (From the New York Evening Post.)

scale, and on Reaumur's; by turning to the number 200 in the

column headed Degrees of Fahrenheit, you find in the hori. The human mind, -that lofty thing!

zontal line with 200, and in the other two columns to the
The palace and the throne,

right, the numbers 93.33 and 74:66, which indicate that the
Where reason sits a sceptred king,
corresponding degrees of the given temperature are 930-33 on

ng And breathes his judgment tone.

the Centigrade scale, and 74° -66 on Reaumur's.
Oh! who with silent step shall trace

Again suppose that when the temperature of a body is 70°
The borders of that haunted place,

Centigrade, and you wish to know what it is on Fahrenheit's
Nor in his weakness own

and Reaumur's scale ; by turning to the number 70 in the
That mystery and marvel bind

column headed Degrees of the Centigrade, you find in the hori. That lofty thing,—the human mind!

zontal line with 70, and in the other two columns, one to the The human heart,- that restless thing!

left and one to the right, the numbers 158 and 56, wbich indiThe tempter and the tried ;

cate that the corresponding degrues of the given temperature The joyous, yet the suffering, –

are 1589 on Fahrenheit's scale, and oto on Roaumur's.
The source of pain and pride;

Lastly, suppose that when the temperature of a body is 59°
The gorgeous thronged,-the desolate,

Reaumur, and you wish to know what it is on the Centigrade
The seat of love, the lair of hate,

scale and on Fahrenheit's; hy turning to the number 69 in the
Self-stung, self.deified !

column headed Degrees of Reaumur, you find in the horizontal Yet do we blees thee as thou art,

line with 59, and in the other two columns to the left, the Thou restless thing,—the human heart!

numbers 73-75 and 164:70, which indicate that the correspond The human soul,--that startling thing!

ing degrees of the given temperatures are 730-75 on the Centi

grade scale, and 164":75 on Fahrenheit's,
Mysterious and sublime !
The angel sleeping on the wing

When the proposed number of degrecs on Fahrenheit's ther

mometer exceed the limits of the table, that is, are above the
Worn by the scoffs of time,
The beautiful, the veiled, the bound,

boiling point, then the rules given in our last Lesson must be
The earth-enslaved, the glory-crowned,

employed, in order to convert them into the corresponding

number on the other two scales. Thus, suppose that the
The stricken in its prime!
From heaven in tears to earth it stole,

temperature of a body, as indicated by Fahrenheit's thermo.
That startling thing,—the human soul!

mewr, was 600°, and it was required io tind the corresponde
ing number of degrees on the Centigrade and Reaumur's, we
have the following calculations :-

€0)
• The composition of acetic acid is 4 equivalents of carbon + 3 of hydro-
gen + 3 of oxygen; bence its full symbolic notation is C, H, Oz; bui this
being somewhat inconvenient, chemists usually represent it by its initial

568 letter A with a dash orer it, thus Ã; and here it may be remarked that a dush over a letter siznities the body to be of the organic kingdom, i.e. either of animal or vegetable origin, whereas a dash under an initial is a

9)2845.00 reduplication of quantity. Thus , means acetic acid, and A 2 of acetic acid.

315-35 Centigrade.

202-41 Reaumurit:

1

6000

32

668

5

4

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78.88 78 75 78.33 78 77.77 77.5 77.22 77 76.66 76.25 76.11

58 57.77 57-5 57 22 57 56.66 56.25 56:11 56 55.55

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54.44
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63.88
53.75
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52 77
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51.66
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98.88
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97.77
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96.66
96.25
96.11
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95.55
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94.44
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93.88
93 75
93 33
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92 77
92 5
92 22
92
91.66
91.25
91.11
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90
89.44
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88.88
8875
89.33
88
87.77
87.5
87.22
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86.66
86.25
86 11
86
85.55
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81:44
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83 88
83.75

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82.77
82.5
82.22
82
81.66
81.25
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80 55
80
79.44
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174
173 75
173
172:4
172
171.5
171
170.6
170
169 25
169
168 8
168
167
166
165 2
165
164.75
161
163.4
163
1625
162
161.6
161
100.25
160
159 8
159
158
157
136.2
155
155.75
155
151:4
154
163.5
153
152 6
152
161.25
151
150.8
150
149
148
147.2
147
146.75
146
1.15.4
145
141.5

121

80
79 55
79.2.
79.11
79
78.66
78.4
78.22
78
77.77
77.6
77.33
77
76.88
768
76.44
76
75'55
75.2
75.11
75
74.66
74.4
74.22
74
73.77
73.6
73.33
73
72.88
72.8
72.44
72
71.55
71.2
71.11
71
70.66
70.4
70.22
70
69.77
69 6
69.33
69
68.88
68.8
68 41
68
67 55
672
67.11
07
66.60
66.4
66.22
66
65-77
65-6
65 33
65
64.38
618
64.44
64
63.55
63.2

43 55
43 2
43:11
43
42 66
42.4
42.22
42
41.77
41.6
41:33
41
40.88
408
40:41
40
39 55
39 2
39.11
39
39.66
38:4
38.22
38
37.77
37.5
37.33

99
98 6
98
97.25
97
96.8
96
95
91
93.2
93
92.76
92
91 4
91
90.5
90
89 6
89
88 25
88
87.8
87
83
85
81.2
8+
83.75
83
82.4
82
81 5
SI
80.6
80
79.25
79
78.8
78

119

63:11 136.4 63 136 62.66 135.5 62-4

135 62.22 1346 62

131 61.77 133 25 61.6 133 61.33 132 8 61 132 60.88 131 60 8 130 60 44 129 2 60

129 59.55 128-75 59.2 128 59:11 127:4 59

127 58.66 126.5 58.4 126 58.22 125.6 58 125 57.77 124.23 57.6

124 57.33 123.8 57 123 56.88

122 56.8 56'44 120.2 56

120 55.55 119.75 55.2 55.11 118.4

55

75.55
75
74.44
74
73 88
73.75
73.33
73
72.77
72.5
72.22
72
71.66
71.25
71.11
71
70-55
70
69:44
69
68.88
68.75
08:33
68
67.77
67.5
67-22
67
66.66
66.25
66.11
66
65.55
65
64.44
64
63.83
63.75
63.33
63
02-77
62.5
62.22
62
61.66
61.25
G1:11
61
60-55
60
59:44
59
58.88
53.75
58.33

37

189

76 75.2

36.88
36.8
36.44
36
35 55
35.2
35:11

49:44
49
48.88
48.75
48.33
48
47.77
47.5
47.22
47
46 66
46.25
46:11
46
45.55
45
44.44
44
43.88
43.75
43:33
43
42:77
42.5
42.22
42
41.66
41:25
41:11
41
40.35
40
39.44
39
38:88
38.75
38:33
38
37.77
37.5

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31.66
34 4
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33.77
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32 88
32.8
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118 54.66 117.5 54.4 117 54.22 116.6 54

116 53.77 115.25 53.6

115 53.33 114.8 53 114 52.88 113 52.8

112 52.44

111.2 52

111 51.55 110.75 51.2 110 51.11 109.4 51

109 50.66 108.5 50 4 108 50.22 107 6 50 107 49.77 106.25 49.6 106 49.33 105.8 49 105 48.88 104 48.8

103 48:44 102.2 48 102 47.55 101.75 47.2

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37.22
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36.66
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34:44
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32.22
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30.55
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29 44
29
28 88
28 75
28 33
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27 77
27.5
27.22
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26 63
26.25
26:11
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25.55
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21:44
24
23 88
23 75
23:33
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22-77
22:5
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21.66
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19.41
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18.33
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29.6
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28 88
28.8
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27 55
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48.2
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47.75
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43.25
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42.8
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12.88
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12:44
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9.6
9.33
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8.38
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6.4
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5.6
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4.8
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2.4
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33.8
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32
31
30.2
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29 75
29
28:4
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27.5
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26.6
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25.25
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24.8
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23
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21.2
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20.75
20
19.4
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18
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16.25
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15.8
15
14
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12.2
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11.76
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-4.8
-4.88
-5
-5.33
-5.6
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-12-77
-13
-13:33
-13.75
-13.88
-14
--14'44
-15
-15.55

16
--16:11

- 16.25
- 16.66
-17
-17.22
-17.5
-17.77
-18
-18.33
-18.75
-18.88
-19
- 19.44
-20
--20:55
-21
-21.11
-21.25
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-22-5
-22-77
-23
-23:33
-23.75
-23.88
-24
-24.44
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-26.25
-26.66
-27

- 10.22-17

10.4 -17.5 -10.66 --18

--18.4 -11.11 -19

- 11.2 -19.75 -11:55

--20 -12 -20.2 - 12:44 -21 -12.8 -22 -12:88

-23 -13 -23:8 --13:33

-24 -136 -24.25 -13.77 -25 -14 -25.6 -14.22 -26 -14'4 -- 26.5 --14.66 --27 -16 --27.4 -16.11 -28 --16.2 -28.75 --15.55

--29
-16 -29-2
-16.44 -30
-16.8 -31
-16.88 -32
-17 --32.8
-17.33

-33
-17 6 -33.25
-17.77 -34
-18 -34 6
-18.22 -35
-18 4 -35-6
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-36
-19 -36:4
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-19.2 -37.75
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-20.44 -39
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-40
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-41.8 --21.33 -42 --21.6 -42.25

- 27.22 -21.77
-27.5 --22
--27.77 --22:22
-28 -22:4
-28.33 -22-66
-28 76 -23
-28.88 -23:11
-29 --23:2
29-44 -2355
-30

- 24
-30-55 24.44
-31 -248
31.11

-24.88
-31-25 -25
-31 66 -25-33
-32

-256
-32-22 -25 77
-325
-32.77

-26 22
-33 -2654
-33.33 -26.66
-33.75 -27
-33.88 --27.11
-34 -27-2
--34.44 --27 56

-35 --28 -35-55-28.44 -36

-28-8 -36 11 -28-88 -36-26 -29 --36.66 -29.33 -37 - 29 6 -37.22--29 77 -37-5 --30 -37.77

--30-22 -38 -30-4 -38.33 --30.66

-38.75 -31 -38.88 31.11

-39 -312 -39'44 -31.55 -40 -32 -40-55 -32 41

-32:8 41.11 -32.88 41.25 -33

-4.44

-5
-5.55
-6
-6.11
-6.25
-6.66
-7
-7.22
-7.5
-7.77
--8
-8.33
-8.75
-8.88
-9
--9:44
-10
--10:55
-11
-11.11
-11.25
-11.66
--12
- 12.22
-12.5

-5.8

8.88 8.75 8.33 8 7.77 7.5 7.22 7 6.66 6.25 6:11 6 5.55 5 4.44

--6.25

-706
-8
- 8.5

-9
-9.4

-10

-6.22
-6.4
-6.66
-7
-7.11
--7.2
-7.55
- 8
-8.44
-8.8
--8.88

-9
-9.33
-9.6
-9.77
-10

-10.76
-11
-11.2
-12
-13
-14
-14.8
-15
-15.25
-16
--16-6

3.88 3.75 3.33 3 2.77 2.5 2.22 2

-41

-21

THERMOMETERS.

primitive state of aggregation, this explanation being founded

on the supposition that, at the end of two or three years, the Displacement of the Standard Point. - Thermometers, con- fixed point is no longer subject to displacement. But, accord. structed even with the greatest care, are subject to one cause ing to the experiments of M. Despretz, this displacement of error which it is important to observe; it is this : that after appears to continue during a period almost indefinite. Besides a certain time, the standard point, which is zero on the cen- this slow displacement, sudden variations in the position of tigrade, and 32° on Fahrenheit's thermometer, tends to rise, the fixed point have been observed, whenever the thermo: 80 that sometimes it is found out of its place by as much meter has been raised to a high temperature. Indeed, if at as 20 on the former, and nearly 4o on the latter; that is, that time we immerse the instrument in melting ice, the the mercury no longer descends to the fixed point of the scale, mercury sinks no longer to the freezing point on the scale, when the thermometer, is immersed in melting ice. Various and it returns to it only at the end of a certain period. It is explanations of this phenomenon have been given, but none of importance, therefore, when it is required

to measure temo appears to be quite satisfactory. It has been attributed to a peratures with precision, frst to verify the position of the diminution of the volume of the bulb, arising from the effect fixed point in the thermometer which is about to be employed. exists in the thermometer; but it has been observed that ters which are accurate with respect to the two fixed points thermometers which contain air change like those which on the scale, namely, the freezing and the boiling points, are bulb, after it has been blown, returns bat very slowly to its times vary by several degrees. M. Regnault thinks that this

differenee is owing to the unequal expansion of the glass of liquid is introduced into it, in sufficient quantity to fill the
which the thermometers are constructed. These different horizontal branch, and half of each of the vertical branches
remarks show that the determination of temperatures presents of the tube. It is necessary to select a liquid for this purpose
chances of error, and that the greatest care must be taken in which emits no vapour at ordinary temperatures ; hence, in
the management of the thermometer.

general, sulphuric acid coloured red is employed. The appa-
Limits in the use of the Mercurial Thermometer.--Of all the ratus being thus completed, the air is made to pass from one
thermometers constructed on the principle of the expansion of bulb to the other, by heating them unequally, until, after
liquids, preference ought to be given to the thermometer of several trials, they are brought to the same temperature, and
mercury, because that this liquid expands in the most regular the level of the liquid becomes the same in both branches.
manner; it has even been observed that its increase in volume, The mark for zero is then put at each extremity of the liquid.
between 36° and 1000 centigrade, that is, -32008 and 212° It is then graduated by raising one of the bulbs to a tempera-
Fahrenheit, is proportional to the intensity of the heat. Yet, ture 10° higher than that of the other, when the air which it
for temperatures lower than -36° C. or --32°.8 F. it is neces- contains expands and forces the liquid column to rise in the
sary to have recourse to the alcohol or spirit-of-wine thermo- other branch; when this column becomes stationary, 10 is
meter; for mercury freezes at -40° Centigrade, or --400 Fah-marked on each branch at the point which the lovel of the
renheit ; and in approaching this point its expansion is irre- liquid has reached; then the intervals from po to 10o are
gular, that is, not proportional to the intensity of the heat. divided into 10 equal parts, and the divisions are carried above
In the case of high temperatures, the indications of mercurial and below zero along each branch of the instrument.
thermometers cannot be made to exceed 350° Centigrade or The Thermoscape of Rumford.At the same time that Leslie
662° Fahrenheit, because this is reckoned the boiling point of invented the Differential Thermometer, the American citizen
mercury; but, according to Mitscherlich, it is 360Centigrade Count Rumford (alias Benjamin Thomson), who died at Aute-
or 680° Fahrenheit.

uil, near Paris, in 1814, invented a thermometer of a similar
The Alcohol Thermometer.-The alcohol or spirit of wine nature, which has been called Rumford's Thermoscope. The
thermometer differs from the mercurial thermometer only in difference between this instrument and the preceding one
the liquid employed; the alcohol is coloured red with car- consists in this, that the two bulbs are larger, the horizontal
mine. But the expansion of liquids is less regular when branch is longer, and it is upon this branch that the gradua-
they approach their boiling point; thus alcohol, which boils tion is made, see fig. 162. The index i is only about thre-
at 172° Fahrenheit, expands very irregularly between the
two fixed points of the thermometer. Thus, after having

Fig. 162.
determined these points, as for the mercurial thermometer,
if the interval between them on the alcohol thermometer be
divided into 180°, it will be found that the latter thermometer
does not give the same indications as the mercurial one be-
tween these points : in fact, the indications will be less by
several degrees, so that when the mercurial thermometer
stands at 122° Fahrenheit, the alcohol thermometer will stand
at 111. Fahrenheit. In order, therefore, to construct an accurate
alcohol thermometer, it must be done by comparison with
a mercurial thermometer as the standard ; that is, the two
thermometers must be gradually heated together in a bath,
and the temperatures indicated by the mercurial thermometer
must be marked on the alcohol thermometer at the successive
levels shown by the latter liquid. Thus graduated, the alcohol
thermometer is equivalent to the mercurial thermometer, that
is, it marks the same temperatures when it is placed in the
same conditions. The alcohol thermometer is particularly
employed to measure low temperatures, because that this
liquid is not frozen by the greatest known degrees of cold.

Leslie's Differential Thermometer. - Professor Leslie (after. quarters of an inch in length, and zero is marked at each ex-
wards Sir John), who died in 1832, a Scotch philosopher, tremity of it when, the two bulbs being at the same tempera-
constructed an air thermometer, intended to show the difference ture, the index occupies the middle of the horizontal branch.
of temperature between two material points or media in the The rest of the graduation is conducted on the same principle
close vicinity of each other; hence its name, the Differential as in the preceding instrument. As to the appendage D, it is
Thermometer. This instrument is composed of two glass intended to regulate the apparatus, so that when too much
bulbs filled with air, and connected with one another by a air gets into one of the bulbs, the index is passed into this
narrow tube twice bent at right angles from the extremities appendage, and the air is then allowed to pass into the other
of its horizontal branch, and fixed on a stand, as shown in fig: bulb. In order to make the index come out of this appen-
161. Before the apparatus is hermetically sealed, a coloured dage and take the position which it ought to occupy, it is
Fig. 161.

sufficient to incline the thermometer to one side ; but this is
effected only after several trials. There are other kinds of ain.
thermometers, to which we shall refer in the sequel.

Breguet's Metallic Thermometer.-A thermometer constructed
on the principle of the unequal expansibility of metals, and
remarkable for its extreme sensibility, was invented by A.
Breguet, a watchmaker of Paris, who died in 1823. This
instrument is composed of three equal slips of metal placed
one above another, namely, platinum, gold, and silver, and
soldered together throughout their whole length; they are
then passed through a flatting machine, so as to form a very
thin metallic ribbon. This ribbon is wound in a spiral form
round a support, as shown in fig. 163, and the upper extre-
mity being fixed on it, there is attached to the other extremity,
which is free, a slight needle or index, which is also free
to move over a horizontal dial carrying a centigrade scale.
The silver, which is the most expansible of the three metals,
forms the interior face of the spiral ribbon; the platinum,
which is the least expansible, forms the outward face; and
the gold is between them. When the temperature is raised,
the silver expanding more than the platinum or the yord, the

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