come within the sphere of our cognisance. These properties diagram this correspondence occurs at the eighth division or are distinguished into general and special. The former are the vernier, counting from the point N. This coincidence those which belong to all bodies, of whatever kind and in what-shows that the fraction to be measured is equal to eight-tenths. ever state they may be examined. The properties necessary to | In other words, the divisions on the vernier being smaller than be considered at this time, are impenetrability, extension, divisi- than those on the fixed rule by one-tenth, it follows that if we bility, porosity, compressibility, elasticity, mobility and inertia. begin to count at the point of coincidence, and proceed in the Special properties are such as are observed in certain bodies, direction from right to left, each successive degree on the veror under certain physical conditions. Of this kind are solidity, nier falls in arrear of the corresponding degree on the fixed fluidity, tenacity, ductility, malleability, hardness, transparency, rule by one-tenth. Hence it follows, that in the case under colour, &c. For the present we shall only be concerned with consideration from the extremity N of the vernier, to the fourth the general properties of matter already mentioned; but it is division on the fixed rule, the intervening space is eight-tenths, proper to remark that impenetrability and extension, are not and we arrive at the final conclusion that the length of the so much to be regarded in the light of general properties of object м N to be measured, is equal to four of the divisions of A B matter as the essential attributes of matter itself, and which plus eight-tenths. Consequently if the divisions on the great or serve to define it. Furthermore we may here remark, that fixed rule are hundredths of inches the length of M N will be the terms divisibility, porosity, compressibility, and elasticity obtained almost exactly correct to one-thousandth of an inch. only apply to bodies regarded as made up of aggregated mole- Were it desired to be still more accurate, to obtain the length cules; they are inapplicable to atoms. correct to the two or three thousandth part of an inch, it would then be necessary to divide AB into hundredths of an inch, to cut off the vernier rule until its length should be equal to nineteen or twenty-nine divisions of the great rule, as the case might be, and finally to divide the vernier into twenty or thirty equal parts. But when such minute divisions as these have to be observed, and the exact line of coincidence between the degrees of the vernier and the fixed rule accurately read off, the aid of a lens is absolutely necessary. The vernier is not invariably a linear duated circular arcs are supplied with verniers, which are then usually engraved in such a manner that fractions of a degree are read off in minutes and seconds. It may be proper here to remark that the vernier is also occasionally termed a nonius, and still more frequently in mathematical books of a past era, the nonius vernier. It derives this name from Nunez, a Portuguese mathematician, who is considered by some to have been its inventor. This, however, is not the case. instrument of Nunez, although designed for accomplishing a similar purpose with the vernier, differed from it in some important respects, and was far less efficient.

Impenetrability. This is the property by virtue of which no two material elements can simultaneously occupy the same point in space. This property, strictly speaking, only applies to atoms. In a great number of cases, bodies appear to be susceptible of penetration. For example, there exist certain alloys, of which the volume is less than the joint volume of the metals entering into their composition. Again, on mixing water with oil of vitriol or with alcohol, the mixture contracts in volume. Such phenomena do not represent actual penetra-measure, as we have already described it; very frequently gration. The appearance is solely referable to the fact, that the materials of which the acting bodies are composed are not in actual contact. Certain intervals exist between them, and these intervals are susceptible of being occupied by other matters, as will be demonstrated further on, when we come to treat of porosity.

Extension. This is the property which every material body possesses of occupying a limited and definite portion of space. A multiplicity of instruments has been constructed, having for their object the measuring of space. Amongst these the vernier and the micrometric screw are very important; we will therefore proceed to their consideration. The Vernier is so called from the name of its inventor, a French mathematician, who died in 1637. This instrument enters into the construction of numerous kinds of apparatus used in the study of the physical sciences, such, for example, as barometers, cathetometers, goniometers, &c. It is composed of two engraved rules, the larger of which A B (fig. 1), is fixed and divided into equal parts. The smaller rule is moveable, and to this in strict language the term vernier is alone applicable. To graduate the vernier, the process is as follows. First of all it is cut to such a length as corresponds with nine divisions of the large or fixed rule. It is then divided into ten equal parts, from which arrangement it follows that every division of the rule a d is smaller than a division of the rule A B by one-tenth.

The vernier being thus constructed as already described, let us explain the manner of its application. Suppose it was desired to measure the length of an object м N. We place it as represented in the figure upon the great rule, the long axis of which corresponds with that of the body to be measured, and we find that its length equals four units plus a certain fraction. To value the amount of this fraction is the object of the vernier. This is accomplished by sliding the vernier along the length of the fixed rule, until the end of the vernier corresponds with the end M N of the object to be measured. This adjustment being made, we next seek for the point of coincidence between the divisions of the two rules. In the

The

The Micrometric Screw and Dividing Machines. The term micrometric is applied to that variety of screw employed for measur ing with precision the extension of length and breadth. It follows, from the very nature of a screw, that when it is well and accurately made, its pitch, or the interval existing between any two successive threads, must be everywhere throughout its length the same. From this it follows, that if a screw be rotated in a fixed nut, the former will advance a certain equal distance for each revolution, the rate of advance being proportionate to the degree of obliquity of the screw-thread. It follows, moreover, that for every fraction of a turn, say th, it only advances the 18th of the length of an interval between any two threads. Consequently if this interval be equal to a hundreth of an inch, and if at the handle extremity of the screw there is attached a wheel or circle graduated into 400 divisions, and turning with the screw, then on turning the graduated wheel through only one division, the screw itself will be caused to advance to the extent of one 400th of an inch.

Dividing machines, as they are termed, depend on the application of this principle. Fig. 2 represents a dividing machine, intended for the division of straight lines. It is composed of a long screw, the thread of which ought to be perfectly regular, working through a fixed metallic plate, and its handle part attached to a fixed metallic circle A. Adjacent to this graduated wheel is attached a fixed index B,-by means of which every fraction of a turn made by the wheel, and consequently the screw itself, may be easily discriminated. The nut E, through which the screw plays, is attached to an iron rule CD, which moves with the nut by a motion parallel to the axis of the screw. It is upon this rule which is fixed the object m n intended to be divided. Lastly, the table is supplied with two brass grooves perpendicular to D C, and upon which moves the slide-rest K, armed with the steel graver o.

The machine being arranged according to the description just given, two different cases may present themselves. Either the rule mn has to be divided into equal parts of a determinate length-for example, four hundredths of an inch-or it may have to be graduated into a given number of equal parts. Under the first conditions, the course of the screw, or its length from thread to thread, being equal to one hundredth of an inch, the

LESSONS ON CHEMISTRY.

operator turns the circle a through one-fourth of an entire revo-
lution, engraves a mark on the rule, then turns the wheel through
another fourth of a revolution, engraves another mark, and so
completed. Under the second
proceeds until the operation
conditions, let us suppose the division of the rule mn into
The
eighty equal parts to be the problem for solution.
operator now commences by causing the screw to turn in
the direction from right to left, as relates to our diagram,
until the extremity m exactly coincides with the point of the
graver; then reversing the direction of rotation, and causing
the wheel to move from left to right, in relation to the diagram
until the other extremity n of the rule corresponds with the
point of the graver. The operator counts the number of turns,

substance in an apartment the air of which is frequently
renewed.

Another example of the extreme divisibility of matter, even
when organised, is furnished by the globules of the blood.
Blood is made up of red globules, floating in a liquid termed
serum. In man, these globules are spheroidal, and their dia-
meter only amounts to about the 0003th part of an inch.
Nevertheless, the particle of blood capable of being taken up
on the point of a needle contains nearly 1,000,000 of such
globules. But, what is more wonderful still, certain animals
They move about as large
exist so amazingly small, that they can only be seen by the aid
of a microscope of high power.
animals do; they are nourished; they possess organs; how
Fig. 2.

and the value of the fraction of a turn, if such exist, gone through by the graduated wheel in causing the rule CD to advance from one extremity of the object mn to the other. Then, dividing the total number of revolutions by 80, the quotient indicates the space along which the screw E must advance for each oth of mn. It only now remains to engrave a mark on mn at the cessation of each partial revolution of the wheel.

Divisibility. This is the property which all bodies possess of being susceptible of division into distinct parts. Numerous examples might be cited illustrative of the extreme divisibility Thus one grain of musk is sufficient to evolve during many years the peculiar odorous particles of that

of matter.

The four-ounce bottel with its tobacco-pipe attachment, will not be required just now, but we shall speedily want it, therefore let the arrangement be made at once. Now the treatment of the cork involves two separate processes, boring and external fitting, and the order in which these operations are performed is not immaterial. The boring operation must come first. There are two methods of boring a cork; either by thrusting a pointed red-hot wire through it, and afterwards accurately enlarging the orifice by means of a rat's-tail file, or by the use of a special instrument termed a cork-borer. The

immeasurably small must those particles be of which such animals are composed!

The divisibility of any kind of matter having been pushed so far that its particles are altogether imperceptible, even by the aid of the most powerful microscope, experiments can Nevertheless, the stability of chemical no longer determine whether such matter be finitely or infinitely divisible. properties belonging to each kind of matter, the invariability of relation subsisting between the weights of combining elements, and other important considerations, point to belief in a finite limit to material divisibility. Circumstances of this kind have led philosophers to assume that bodies are constituted of material elements not susceptible of division, and to which, therefore, the term atoms is applied.

latter is by far the more convenient plan of the two. I have not assumed the student to possess a cork-borer, but I will describe the instrument, so that it may be made or procured at once if convenient. It merely consists of a piece of brass tube, such as is employed for the ferrules of fishers' rods, of equal size with the hole to be bored, and sharpened by filing to a rough saw-edge at one end. If a transverse hole be bored through the brass tube towards the other end, all the better: the con

Fig. 2

T

trivance permitting the insertion of an iron wire as represented by a, thus attaching to the instrument a sort of gimlet handle, and conferring that kind of additional power which mechanics term for the sake of brevity "purchase,"-with such an instrument as this, cork-boring is a very simple affair. A cork-bore,

Fig. 4.

being taken of the proper diameter, its edge is sharpened by a [ it remains to attach the length of India-rubber tubing to the few rubs of the file, and pressed against the cork under con- tobacco-pipe shank, and a few inches of glass tubing to that of tinuous rotatory motion, when it soon penetrates through the India-rubber, so that eventually an apparatus may result of the central core, escaping through the tube itself. As there is following shape, where a represents the point of attachment some little chance, however, that the side of the cork where the between the India-rubber tube, and tobacco-pipe shank; and hole emerges may assume a ragged aspect, it is better to commence the operation at one end of the cork, then without penetrating quite through withdraw the borer, and recommence at the other end, thus causing the operation to terminate in the middle. If the aperture be clean and smooth it may be considered finished; if it be rugged and uneven, how. ever, it will require trimming with the rat's-tail file. The aperture being made, we now come to the insertion of the tobacco-pipe shank, a matter of much simplicity; one would think that no special instructions were necessary. It is not 80:-the operation requires to be set about in a systematic way; and although in this case, the operator might succeed after many attempts, and tobacco-pipes being cheap enough, these numerous attempts might be made without the objection of great expense; yet considering the necessity for performing similar operations under modified circumstances to which the objection of expense and many others would strongly apply, it is better to cultivate the right habit at once. Remember, then, tobacco-pipes and glass tubes are not like metal rods. We cannot fit them tightly, by violently twisting, turning, and pushing, nevertheless we must fit them air-tight. Our object is accomplished by easing them in, to use a popular but an expressive word. Their accuracy of adjustment is secured by paying attention to various little circumstances of detail. If, then, the end of the tobacco-pipe shank be ragged, as it most likely will be, rub off those ragged inequalities by means of a file. Had we been concerned with a thin glass tube instead of a tobacco-pipe, the better plan of treatment would have consisted in melting the extreme end of the same by holding it for a few instants in the flame of a spirit-lamp or a jet of gas.

Fig. 3.

a' the point of attachment between the latter, and the associated glass tube. Perhaps it is scarcely necessary to indicate that round or oval glass flasks will not stand upright without some kind of support; they may require to be supported whilst exposed to heat or after removal from heat. In the former case rings or triangles are usually employed, attached to a vertical stand, and capable of elevation or depression (fig. 5). Instruments of this kind can be procured ready made, but every experimenter possessed of moderate ingenuity can prepare them or their substitutes for himself. A carpet-rod, around one extremity of which has been cast a block of lead, answers perfectly, and the rings may be made of stout iron wire, as represented in fig. 6.

Fig

Our present operations having reference to clay, not glass, we have not this resource; but on the other hand a tobaccopipe shank is stronger than a glass tube, in consideration of which I have chosen it, otherwise a piece of glass tube would have answered the purpose equally well.

Having finished the attachment of the tobacco-pipe shank, we now come to the attachment of the cork itself, which is effected by accurate filing, a slightly conical form being imparted to the cork, in order that it may tightly fit with the minimum of pressure. This precaution is especially requisite when a thin necked flask has to be corked. In this case a very slight amount of pressure will infallibly break the neck

of the flask.

An examination of the mechanical conditions to which the wire ring is subjected will prove that it requires no screw or other contrivance for fixing, when moderate weights have to be supported.

Matters are now ready for the commencement of our operations. The subject of this lesson is zinc, but it is iron which must first claim our attention. We require to effect a combination of this metal with sulphur, in order that something may be made wherewith certain properties of the zinc may be tested. The combination of sulphur with iron is called sulphuret of iron, occasionally the sulphide of iron, and let the

reader well remember that

A SULPHide

or

A SULPHUret

}

is not

{

A SULPHIte

or a

A SULPHate.

The cork I will now assume to have been accurately adapted, by filing, to its orifice; but it is hard and rigid. Corks may be softened by immersion in boiling water, a treatment which will answer all present ends; but cases frequently present themselves when a cork, forming part of a chemical apparatus, must be absolutely dry, under which circumstances it must be the termination ide or uret express the same compound, but the softened by immersion in hot sand, or more extemporaneously, terminations ite and ate express two different compounds; dif but less rapidly, by holding it for a few seconds in the flame ferent not only as materially between themselves, but as of a spirit-lamp.

Having completed the arrangements to the extent described,

LESSONS IN CHEMISTRY.

between themselves collectively and a sulphuret or sulphide. | a proper substitute must be found to take its place, and hence
What is the difference? No matter. That point will come under the terms water-bath, oil-bath, &c.
consideration by-and-by; we are not now treating of sulphur
compounds, but of the metal zinc. If the collateral facts just
mentioned choose to attach themselves to the learner's memory,
well and good; if not, let them pass, they will be made to
attach themselves in the sequel. Sometimes, however, when
one gives a collateral fact on the understanding that it may stick

in the brain or take flight just as best suits its own good pleasure, it sticks there all the firmer. I always give collateral facts an option of this kind. To effect the union of sulphur with iron, in other words, to make sulphuret of iron, it is merely necessary to bring a white-hot bar of iron in contact with a roll of sulphur; then the iron drops into melted globules which seem like iron itself, but which in reality are a compound of iron and sulphur, and weigh heavier than the iron by the weight of the sulphur wherewith they have combined. The greater number of metals can be made to combine with sulphur, by a similar treatment to that now described, and, indeed, perhaps the act or combination just effected may have presented itself to the reader's attention under the aspect of natural magic. To melt a nail in a walnut-shell, is a proposition often constituting the subject of a wager. The learner now sees how that wager might be won. A nail being heated to whiteness, is dropped into a walnut-shell containing sulphur, when the fusion of the nail immediately takes place.

Let the sulphuret of iron thus resulting be transferred to a bottle labelled Sulphuret of Iron, and put away,-we shall require it presently. We will now return to the zinc solution, which has been so long neglected that the student may fear the original subject of the lesson has been forgotten. Not so. Every point expatiated on, everything done, has had reference to the metal zinc.

I have already said that the metallic zinc employed remains in the solution; the next point, then, is to ascertain the conditions it has assumed, and this information may be obtained by driving off the liquid in which it is dissolved. This is accomplished by the application of heat, which, causing the liquid to become steam or vapour, the latter is driven off, and all bodies contained in the liquid, not capable of assuming this vaporous condition, necessarily remain.

The application of heat in many processes of evaporation and distillation requires many precautions. For the most part naked fires are ineligible; frequently a sand-bath is the best means of applying heat, and it is the source of heat we shall employ now, fig. 7; but occasionally the heat capable of being imparted by and would be injuriously high, hence

Supposing the solution of zinc in oil of vitriol and water to purpose, under the name of evaporating dish; supposing the be placed in a saucer or porcelain dish, specially made for the solution and its dish to be embedded in the sand-bath, and the latter placed on its hoop-iron tripod over a fire, heat will rapidly penetrate the sand, and evaporation will ensue. the solution were to be evaporated very slowly, the saucer or pan would eventually contain white crystals. If, however, the evaporation be more rapidly pushed, then crystals do not be the case. As soon as evaporation is complete, and the appear, but a white confused mass. I suppose the latter to residue has become thoroughly dry, remove the saucer from the sand-bath, allow it to cool, and when cold dissolve the evaporated material in distilled water. The liquid now returns to the state in which it originally was before evaporation, with this difference, any excess of oil of vitriol over and above the quantity necessary to dissolve the zinc, has been driven away

by evaporation. Pour the solution now into a wine-glass, and | of the zinc has been effected, is a very offensive gas. It is, proceed as follows: