Now, as stated above, the action of the various reagents upon chemical substances has been strictly observed, and from their behaviour a set of laws has been made, by observing which the analyst is enabled to detect the presence, or declare the absence, of particular elements in the subject of his analysis.

Thus, a stream of sulphuretted hydrogen gas, on being passed through a solution of, say, a dozen elements, will throw down six of them in an insoluble form, having made fresh combinations with them, and will leave the other six as they were before. Here at once is a medium for the separation of certain elements from certain other elements; and there are other reagents, which will be spoken of later on, under the head of analysis, by which further subdivisions can be made, so as eventually to disintegrate the whole compound. Certain reagents, acting upon certain sets of elements in a special way, are noted as the special reagents for the elements in those sets, distinguishing them from other groups; and therefore it has been the practice of some chemists to divide the bases into groups (five), each one of which is marked by some particular reagent. I will specify these groups presently, but in the instructions which will be laid down for guidance in making analyses, I shall follow the rule of the Giessen Laboratory (Baron Liebig's), which, taking advantage of the distinctive reagents which will be mentioned, yet works upon a broader basis, prescribing the application invariably of the most general reagent (sulphuretted hydrogen) first, and then applying in order successive special reagents, till the analysis has been pursued to the end.

The following instructions must be observed strictly in the conduct of the analyst:

First, thorough cleanliness of all apparatus. Many an experiment has failed, and many a test has proved deceptive, because some remains from former experiments, or some impurity contracted elsewhere, has been allowed to be in the test-tube, or whatever apparatus is in hand. The table cannot be free from stains, but it can and should be kept dry and quite free from chemical or other refuse. Common water may be used to rough-wash apparatus, but test-tubes should always have a final rinse with distilled water before being used.

Secondly, playing with the chemicals, by which I mean using them without some distinct object in view, should be looked upon as unworthy of a student of chemistry.

Thirdly, patience and perseverance must both be largely drawn upon by the student. He cannot expect to arrive at perfection suddenly in the pursuit of this most patience-taxing, as well as most interesting, study, any more than he can expect to do so in any other matter whatever. But the operations of chemistry are so delicate, so very little (an impurity in a test-tube) will throw its working machinery out of gear, that it is above all things necessary that he should not be disheartened, but persevere, even if he finds himself foiled after having taken all the precautions he thought he could to prevent such a result. One thing he must never do: attribute his want of success in any experiment to chemistry, and not to his own fault. Provided with good chemicals and truthful information how to use them, he cannot but see that any coming-short

He

of the expected result must be due to himself, and not to the science. should ever remember the Virgilian advice, “Tu ne cede malis, sed contra audentior ito!" and, with regard to his chemicals and apparatus, he should bear in mind that it is not the good workman who complains of his tools.

Fourthly, it is never to be forgotten that many of the chemicals are rank poison, so that the greatest care must be used to prevent their getting into the hands of those who do not understand the nature of them.

Before proceeding to the subject of analysis, as applied to the separation of metals and earths from each other, let us go through a few experiments of a simple analytical character in connection with gases, which are not only instructive but highly amusing. And first let us try some with the lightest of gases, hydrogen, two parts of which unite with one part of oxygen to form water. To form water? Two invisible gases make up by their union that immense volume of liquid which washes the globe of the earth, in ocean, sea, river, and lake? Yes; two gases, invisible, odourless, only to be felt through effects, combine to do all this, and to make a solvent for those millions of tons of chemical salts which are found in sea-water and river-water, and to get rid of which, by distillation, lest they should confuse the investigation, is so necessary before using the water in the process of analysis. Fire is the medium by which they are united so as to form water; but fire will not suffice to disunite them when once joined, unless at a very high temperature. Electricity dissects water into its two elementary gases, and the same result is obtained in the manner already indicated when describing the hydrogen gas-maker, and is incidental to several other chemical operations.

HYDROGEN (the water generator) may be made either in the manner already indicated, or by heating water till it rises into vapour, when it is passed through a gun-barrel, heated red hot and filled with nails. The steam is decomposed by the heat, and the oxygen of the water unites with the iron to form oxide of iron, while the hydrogen passes through the pipe, and may be secured on its exit. But this will require more apparatus than we are supposed to have; besides, hydrogen is more quickly and conveniently prepared by the zinc and diluted sulphuric acid in the machine.

Having filled the gas-jar in the manner pointed out when describing that article, remove it from the shelf in the water-bath into the air. The gas will not escape so long as the jar is kept mouth downwards, because hydrogen, being lighter than air, will press against the interior sides of the bottle, remaining there until displaced by the atmospheric air. Invert the bottle containing the gas, and apply a lighted taper or match to the mouth of it. It will then be seen how very inflammable this gas is: it will immediately take fire with an explosion, and in course of burning will re-unite with the oxygen of the air, from which it seems not to like to live apart, producing, as will be found on inspection of the sides of the jar, drops of water, which will gather and roll down to the bottom. This experiment is both analytical and synthetical. In the first place, an analysis of the water was made through the action of the acid and zinc upon it, and then a reunion of the elements was made, resulting in the production of water.

This is the rationale of the first process as shown by the use of symbols:

The zinc has undergone a change at the expense of the water, which it has robbed of its oxygen to make oxide of zinc, and with the sulphuric acid this oxide of zinc has united to form a salt of sulphate of zinc. The only element which has not been called upon to play its part in this act is the hydrogen of the water, which is given off and received into the jar.

The reverse process, which is accomplished by means of fire, is so by virtue of some force, the characteristics of which are not yet known. One thing only is certain, that by its aid and nothing else, the two elements, oxygen and hydrogen, are re-united in the form of water.

Water will not extinguish the flame of hydrogen. Repeat the experiment of filling the gas-jar with the gas, stand the jar on its bottom, ignite the gas, and immediately pour water into the bottle. The effect will be, not to put the fire out, but to make it more fierce; for the water being poured in displaces the gas, which, running out, brings additional fuel to the flame.

A small balloon, made of goldbeater's skin (it costs half a crown to buy it), when filled with the gas, will rise to the ceiling of the room. This is an interesting experiment; but the cost of the balloon makes it an expensive one, and it is doubtful if the student would be able to make an efficient balloon for less money than it would cost to buy it.

Hydrogen is found so largely distributed in nature, that between five and six per cent. of it is found in all animal and vegetable substances. It is not found in combination with the metals. Coal gas, the common illuminating gas, is a compound of hydrogen with charcoal gas, and is called carburetted hydrogen. Combined with chlorine gas (which is obtained from common salt, and is a most dangerous poison when not in combination), it forms the strong solvent and reagent known as hydrochloric or muriatic acid.

OXYGEN. This gas unites with hydrogen, in the proportion of one measure of oxygen to two of hydrogen, to form water. It is the element most largely distributed throughout the world. It is indispensable to life and to combustion, and the poorer the air is in this element, the less healthy it is, and the richer, the more healthy. The benefit of change of air from that of an inland place to the sea-side lies in the greater amount of oxygen in the atmosphere of one place than the other, and the increased appetite which comes on change of residence to the sea-side is caused by the increased combustion, or quicker living, brought about by the larger amount of oxygen with which the subject is in contact. The atmosphere of crowded towns, being loaded with many impure gases, becomes impoverished in respect of this life-giving element; at sea, and near the sea, the atmosphere is not so loaded, but contains a full amount of this most essential article.

To man it is literally the breath of life. The blood which is in man's veins is of a dark red colour, and very impure by reason of its lack of oxygen. Before it can exert that influence over the body which has acquired for it the name of the "life of the body," it must be purified; so it is poured into the heart, in a stream never ceasing till death, and by that little organ is pumped up through the lungs, where, becoming exposed to the air, it is instantaneously purified by the contact: the carbonic acid gas, which made it so dark in colour and so unprofitable, is there breathed out, and the oxygenated blood, bright red, bounding, lively, is pumped into the arteries all over the body, imparting vigour to the whole frame.

Suffocation, whether by strangling or the inhaling of foul air, is no more than the exclusion of oxygen. The life-giver being withdrawn, the life destroyer

carbonic acid gas, is allowed to have sway; the lungs, instead of receiving a supply of oxygen, are choked with an accumulation of carbonic acid-a poison fatal to man (though it is the life of plants)—and death is the result.

To prepare this gas on a small scale, put some binoxide of manganese (MnO2) and some chlorate of potassa (KO,ČIO) into a Florence flask, mixing them well together, and taking care that the flask is quite dry both inside and outside. Into the neck of the flask put a cork with a bent glass tube through it, support the flask on the retort-stand, and put the lighted spirit-lamp at the bottom of the flask; a large quantity of oxygen will be given off, in a sufficiently pure state for our purpose, and it may be received in the stout deflagrating-jar, which will be safer than the pickle-bottle in conducting experi

ments.

FIG. 9.

Oxygen is heavier than air, so that, after removing the jar from the trough (which may be done by sliding it off its shelf under water on to a plate with water in it), the mouth of the jar may be opened without fear of the gas escaping.

Heat a

Into the jar, by means of the iron spoon, introduce a piece of glowing wood, not alight, but red from recent burning. It will burst into a brilliant blaze, so bright as scarcely to be looked at. piece of charcoal before the blowpipe, and do the like. Put a little piece of sulphur into the iron spoon, ignite it, and put it into the jar of oxygen: a wonderfully beautiful light of dazzling brightness will be the result. Care must be taken not to remove the top of the jar in this case till the fumes have subsided, for they are fumes of sulphurous acid, a very troublesome and poisonous gas. Repeat the experiment with iron filings, heated before the blowpipe in the iron spoon; and, taking great care not to touch the phosphorus with the fingers, but only with a pair of scissors or nippers, with a small piece of ignited phosphorus.

FIG. 10.

These experiments are among some of the most beautiful that can be imagined, and may be safely tried, so care and ordinary prudence be exercised.

NITROGEN.--Roughly stated, five volumes of common air contain four of nitrogen and one of oxygen. It is also the principle of fat, and is a constituent of all plants and animals. It is also called azote (a and 【wn), because it deprives of life animals wholly subjected to its influence. It is not ready to combine with other elements. In forced combination with oxygen it forms nitric acid (NO), and it is also largely distributed throughout nature as nitre, or saltpetre, or, more correctly speaking, nitrate of potassa.

It is thus prepared: dip a small piece of sponge in naphtha or spirit of wine, the sponge being fast to a piece of wire, which must be supported in water, so that the sponge shall project 4 in. above the surface. Ignite the naphtha, and at once clap an empty bottle (the pickle-bottle will do) over the burning spirit. After a while (that is, when all the oxygen of the air in the bottle has been consumed by the operation of combustion) the flame will be extinguished, and a quantity of water, proportionate to the quantity of oxygen consumed, will enter the bottle, which else contains nitrogen gas.

Nitrogen will not alone support either life or combustion, the essential of which is oxygen. A lighted taper put into the jar of nitrogen prepared as above will be instantly extinguished, and animals subjected to its influence will die. Few experiments can be made with this gas, on account of its uncongenial nature. It runs exactly counter to oxygen in respect of combustion, and, unlike that gas, avoids all combination whatever.

CARBONIC ACID.-This most deadly poison, when applied to the lungs to the exclusion of all other gases, is widely diffused throughout nature. To plants it is life, as it is death to man; through all their millions of pores the trees suck in this their food, clearing the air of that which to all animals is so noxious. Carbon is in wood, coal, turf, the bodies of all animated creatures, the yield of the earth in the shape of crops; it is in small quantities in the air; and diamonds are pure carbon in a crystallized form, such as Nature only possesses the secret of making. Carbon is, next to oxygen, the most important element in the world.

It may be prepared by pouring any acid on to chalk, which is carbonate of lime. Carbonic acid is very ready to sever any combination it may have entered into, and in this case it will give place to the strange acid, which will make a new compound with the lime, while the carbonic acid is given off. Care should be taken not to inhale this gas, for, if taken exclusively into the lungs, it is a rank poison, not that of itself it has any deleterious qualities, but because it excludes the one thing needful to life, viz., oxygen.

It is considerably heavier than air, and may be poured from one vessel into another, though it is colourless, odourless, and invisible. Put a lighted taper, or even a piece of charcoal that has just been burning in oxygen, into the reservoir of carbonic acid gas, and it will be immediately extinguished. Put a small piece of lighted candle at the bottom of a bottle or jar, and then take the jar with the fire-killer in it and invert it gradually, just as though you were pouring water from it. As soon as the gas reaches the level of the flame, it will extinguish it suddenly and thoroughly.

To show that the breath expired by animals is very fully charged with carbonic acid gas, put some water in which lime has been soaked and then filtered into a glass; blow through a glass tube into the lime-water, when a thick white precipitate will be thrown down-a precipitate of carbonate of lime or chalk. The carbonic acid gas (CO) of the breath has united with the lime (CaO) in the water to produce the salt thrown down.

CHEMICAL ANALYSIS.

Analysis is the highest function of chemistry, and has for its object the separation of compounds into their component parts. The importance of a science which can teach how to examine any substance whatever, so as to pronounce exactly what is contained in it, will be at once apparent. But chemistry can do more than this. She can not only separate the elements of a body, but she can also declare the quantity of each element contained, even to a fractional part of a grain.

The analysis which determines what elements are combined is called qualitative analysis, and that which declares also the proportions in which they are combined is called quantitative analysis. I do not propose to deal with the second branch at all. It is only to be attempted after the other branch has been thoroughly mastered; it requires the utmost nicety of operation, much