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The smallest particle of this salt which has a salt taste, and in general retains the qualities of salt, is the molecule of salt. This molecule, as we know from the specific gravity of the vapor of salt, weighs 58.5 microcriths. We also know by chemical analysis that, in every 58.5 parts of salt, there are 35.5 parts of chlorine and 23 parts of sodium. Hence, a molecule of salt must contain 35.5 microcriths of chlorine and 23 microcriths of sodium, and, in any chemical process in which chlorine gas or metallic sodium is extracted from salt, each molecule must be subdivided into these two parts. Now, both chlorine gas and sodium are elementary substances, and our theory supposes that the numbers 35.5 and 23 represent the relative weights of their atoms. We, therefore, further conclude that the molecule of salt is formed by the union of two atoms, one of chlorine and one of sodium.

In like manner, the molecules of every compound substance are aggregates of atoms, of at least two atoms. each. With the elementary substances it is different. There are many of these whose molecules are never subdivided, and in such cases the molecule and the atom are identical, but there are also several, of which the molecules can be shown to consist of two or more atoms. Thus, the molecules of phosphorus probably consist of four atoms, those of oxygen of two atoms, and those of hydrogen, nitrogen, chlorine, bromine, and iodine, likewise of two.

Assuming that the molecule of hydrogen gas consists of two atoms as just stated, let us dwell on this fact for a moment as explaining our system of estimating molecular weights, which must have appeared, when stated, very arbitrary. You remember that, according to the law of Avogadro, equal volumes of all

gases contain, under the same conditions, the same number of molecules. Then, since a given volume of oxygen gas weighs sixteen times as much as the same volume of hydrogen gas, the molecule of oxygen must weigh sixteen times as much as the molecule of hydrogen; and, if we assumed the hydrogen-molecule as our unit of molecular weight, the molecule of oxygen would weigh sixteen of those units. So, also, as nitrogen gas weighs fourteen times as much as hydrogen, the nitrogen-molecule would weigh fourteen of the hydrogen units. Again, as chlorine gas weighs 35.5 times as much as hydrogen, a molecule of chlorine would weigh 35.5 of the same units. But these numbers, 16, 14, and 35.5, are simply the specific gravities of the several gases referred to hydrogen; so that, if we took the hydrogen-molecule as the unit, the specific gravity of a gas or vapor referred to hydrogen would express the molecular weight of the substance in these units. Instead, however, of taking the hydrogen-molecule as our unit, we selected the half-hydrogen molecule for that purpose, and called its weight a microcrith, thus, of course, doubling the numbers expressing the molecular weights. Ten pounds have the same value as twenty half-pounds, and so sixteen hydrogen-molecules have the same value as thirty-two microcrithis; and thus it is that, with the system in use, the molecular weight of a substance is twice the specific gravity referred to hydrogen.

Now, you can understand the reason why the half hydrogen-molecule was selected as the unit of molecular weight, and made the microcrith. It was simply because the half-molecule is the hydrogen atom. microcrith is simply the weight of the hydrogen atom, the smallest mass of matter that has yet been recog

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nized in science. The hydrogen-molecule consists of two atoms, and therefore weighs two microcriths. The oxygen-molecule weighs sixteen times as much as the hydrogen-molecule, and therefore weighs thirty-two microcriths. The specific gravity of carbonic-dioxide gas is 22, that is, it weighs twenty-two times as much as hydrogen. Its molecule is therefore twenty-two times as heavy as the hydrogen-molecule, and, of course, weighs forty-four microcriths. Hence, in general, the specific gravity of a gas referred to hydrogen is the weight of the molecule as compared with the hydrogenmolecule, and twice the specific gravity of a gas referred to hydrogen is the weight of its molecule in hydrogen atoms or microcriths.

But you will ask: How do you know that the hydrogen-molecule consists of two atoms, and, in general, how can you determine the weight of the atom of an element? This is a very important question for our chemical philosophy, and I will endeavor to answer it in the next lecture.

LECTURE VII.

ATOMIC WEIGHTS AND CHEMICAL SYMBOLS.

As I stated in my last lecture, I am to ask your attention at the outset this evening to a discussion of the method by which the chemists have succeeded in fixing what they regard as the weights of the atoms of the several elements. This method is based, in the first place, on the principle that the molecular weight of a substance can be accurately determined by comparing its specific gravity in the state of gas or vapor with the definite proportions which are invariably preserved in all the chemical processes into which the substance enters. This point has been so fully explained that it is unnecessary to enlarge upon it further.

In the second place, our method is based on the principles of what we call quantitative analysis. I have already stated that the chemists have been able to analyze all known substances, and to determine with great accuracy the exact proportions of the several elementary substances which are present in each. The methods by which these results are reached are, for the most part, indirect, and frequently very complicated. They are described at great length in the works on this very important practical branch of our science, but it would be impossible to give a clear idea

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of them in this connection. It may be well to say, however, that, in order to analyze a substance, it is not necessary actually to extract the several elementary substances and weigh them. Indeed, this can only very rarely be done, but we reach an equally satisfactory result by converting the unknown substance into compounds whose composition has been accurately determined, and from whose weight we can calculate the weights of their elements.

For example, if we wished to determine the amount of sulphur in a metallic ore, we should not attempt to extract the sulphur and weigh it. Indeed, we could not do so with any accuracy; but we should act on a given weight of the ore, say 100 grains, with appropriate agents, and, by successive processes, convert all the sulphur it contained into a white powder called baric sulphate. Now, in accordance with the law of definite proportions, the composition of baric sulphate is invariable, and we know the exact proportion of sulphur it contains. Hence, after weighing the white powder, we can calculate the amount of sulphur in it, all of which, of course, came from the 100 grains of ore.

Evidently, this method assumes an exact knowledge of the amount of sulphur in baric sulphate, which must have been determined previously. This was, in fact, found by converting a weighed amount of sulphur into baric sulphate, and, in a similar way, most of our methods of analysis are based on previous analyses, in which the definite compounds, whose composition we now assume is known, were either resolved into elements or were formed synthetically from the elements.

As the result of such processes as this, we have the relative amounts of the several elements present in the substance analyzed, and it is usual to state the result