WHAT IS A MICROCRITH?

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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 VI.

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 directly inferred from its specific gravity in the state of gas or vapor, the weight of the molecule of any substance in microcriths being equal to twice the specific gravity of the gas or vapor referred to hydrogen. This point has been so fully explained that it is unnecessary for me 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

HOW SUBSTANCES ARE ANALYZED.

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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

in per cents. Thus, the analyses of water, salt, and sugar, give the results stated below:

Understanding, then, that we are in possession of means of determining accurately the weights of the molecules of all volatile compounds, and also the exact per cent. of any element which each substance contains, we can readily comprehend the method employed for finding the weight of the atom. Let it be the weight of the oxygen atom which we wish to determine. We compare all the volatile compounds of oxygen as in the diagram (p. 125). We take the specific gravity of their vapors with reference to hydrogen, and, doubling the number thus obtained, we have the molecular weights given in the column under this heading. The analyses of these substances inform us what per cent. of each consists of oxygen. Hence, we know how much of the molecules consists of this element. The weight of oxygen in each molecule is given in the last column, estimated, of course, like the molecular weights, in microcriths. Having thus drawn up our table, let me call your attention to two remarkable facts which it reveals.

Notice, first, that the smallest weight of oxygen in any of these molecules is 16 m.c.; and, secondly, that all the other weights are simple multiples of this.

Here, certainly, is a most wonderful fact. Remember that these numbers, which are displayed here