we easily represent a molecule by writing together the symbols of the atoms of which it consists, indicating the number of each kind of atoms by figures, as above. A molecule of water, for example, consists of three atoms, two of hydrogen and one of oxygen. Hence, its symbol is H2O. This symbol shows, not only that the molecule consists of three atoms, as just stated, but also that it contains 2 m.c. of hydrogen and 16 m.c. of oxygen. Further, it shows that the molecule of water weighs 18 m.c. If we wish to represent several molecules of water, we place a figure before the whole symbol. Thus, 2H2O represents two molecules of water, 5H2O five molecules of water, etc. Now, since, in all chemical relations, what is true of the molecule is true of the substance, this symbol may be regarded as the symbol of water, and is constantly spoken of as such. Again, a molecule of alcohol is known to consist of two atoms of carbon, six atoms of hydrogen, and one of oxygen. Hence, the symbol of the molecule is C2HO. This symbol informs the chemist that a molecule of alcohol contains 2 atoms or 24 m.c. of carbon, 6 atoms or 6 m.c. of hydrogen, and 1 atom or 16 m.c. of oxygen. It also shows that the total weight of the molecule is 46 m.c. Several molecules of alcohol are indicated by the use of coefficients, as before-thus 3CHO, etc. This is the whole of the system, and you see how beautiful and simple it is. The single letters stand for atoms, and the terms formed by the grouping of the letters stand for molecules, and the very possibility of the system is in itself a very strong proof that molecules and atoms really exist.

Before proceeding to show how admirably this system is suited to express chemical changes, let me ask your attention for a moment to the nature of the

SYMBOL OF ALCOHOL, HOW DETERMINED.

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evidence by which the symbol of a substance is fixed; for, although this evidence is precisely of the same kind as that on which the atomic weights of the elementary substances rest, yet the principles involved are so important that a brief restatement of the evidence, as it. bears on the present problem, seems almost necessary for a clear understanding of the subject. The question is this: What is your proof that the symbol of alcohol, for example, is C,HO, or, in other words, that this symbol represents the constitution of a molecule of alcohol? The evidence is

1. We know by experiment (page 79) that the specific gravity of alcohol-vapor referred to hydrogen is 23. Hence, since, by Avogadro's law alcohol-vapor and hydrogen gas have in the same volume the same number of molecules, the molecule of alcohol is twentythree times as heavy as the molecule of hydrogen gas; and, further, since by assumption the hydrogen-molecule weighs 2 m.c., the alcohol-molecule weighs 46 m.c. 2. We have analyzed alcohol, and know that it has the following composition:

Hence, of the molecule of alcohol 5218 per cent., or 24 parts in 46, consist of carbon, 13 per cent., or 6 parts in 46, consist of hydrogen, and 34,8%, or 16 parts in 46, consist of oxygen. The whole adds up, as you see, 46, showing that we have done our sum correctly.

Analysis, then, proves that, of the molecule of alcohol weighing 46 m.c., 24 m.c. are carbon, 6 m.c. are hydrogen, and 16 m.c. are oxygen. But the weight of an atom of carbon is 12 m.c., hence the molecule con*tains two atoms of carbon, or C2; the weight of an atom of hydrogen is 1 m.c., hence the molecule contains 6 atoms of hydrogen, or H.; the weight of the oxygen atom is 16 m.c., hence the molecule contains one atom of oxygen, or O, and the symbol is C2HO.

Again, why is the symbol of water H2O? 1. The specific gravity of steam referred to hydrogen gas is 9, hence the weight of a molecule of water in microcriths is 18. 2. Analysis shows that water has the following composition in 100 parts:

We know, then, that, of the molecule weighing 18

m.c. of water, 11 hydrogen, and 88

per cent., or 2 m.c., consist of

per cent., or 16 m.c., consist of oxygen. But 2 m.c. of hydrogen equal 2 atoms, or H2, and 16 m.c. of oxygen 1 atom, or O. Hence, the symbol is H2O.

You see how simple is the reasoning and how definite the result; and, unless our whole theory in regard to molecules and atoms is in error, there is no more doubt that the symbol of water should be written H2O, than that this familiar liquid consists of oxygen and hydrogen gas.

But many of my audience will remember that, when they studied chemistry, the symbol of water was

WHY IS H2O THE SYMBOL OF WATER?

139

HO, and will ask, Why this change? I answer: This difference is of a type with the whole difference between the old and the new schools of chemistry. Indeed, the two symbols may be regarded as the shibboleths of the two systems. In the old system, the symbols simply stood for proportions, and nothing else. The symbol H meant 1 part by weight of hydrogen, and O 8 parts by weight of oxygen; and HO meant a compound, in which the two elements were combined in the proportions of 1 to 8, which is as true of water now as it was then. In the old system, the special form of the symbol, whether H2O, HO, or HO2, had no significance, for this was determined by the arbitrary values given to the letters. There is a second compound of hydrogen and oxygen called hydric peroxide, in which the elements are combined in the proportion of 1 of hydrogen to 16 of oxygen; and, had the chemists of the old school assigned to the symbol O the value 16 instead of 8, then the symbol of hydric peroxide would have been written HO, and that of water H2O; and the only reason usually given for making O represent 8 parts of oxygen instead of 16 was, that water, being very widely diffused in Nature, and the most stable compound of the two, ought to be represented by the simplest symbol; or, in other words, that the ratio between the quantities of oxygen and hydrogen, which it contains, ought to be taken as the type ratio between these elements.

This reasoning was as unsatisfactory as it has proved to be unsound. It might justly have been said that the system, although artificial, was consistent in itself, and that it better suited the requirements of the system to assign to oxygen the proportional number 8, than to select a multiple of that number. Indeed, this

was the light in which the whole scale of proportional numbers was regarded by a large majority of the students of chemistry during the first half of this century; and it is only necessary to state that the German chemists, following the lead of Berzelius, used for years a scale in which oxygen was taken as 100, in order to show how purely arbitrary the actual numbers were considered to be. The only truth that the numbers were believed to represent was the law of definite and multiple proportion; and, so long as the true proportions were preserved, any scale of numbers might be used which suited the experimenter's fancy.

It is, however, perfectly true that, in selecting one of several multiples, which might be used for a given element in a given scale, the decision of the chemist was not unfrequently influenced by the very ideas which now form the basis of our modern science; as is shown by the fact that the proportional numbers of Davy and Berzelius were called chemical equivalents by Wollaston, and atomic weights by Dalton and his pupils. But, then, the truths, which these terms now imply, were never fully conceived or consistently carried out. The atomic weights of the new system are the weights of real quantities of matter, the combining numbers of the old system were certain empirical proportions. So is it in other particulars, and the difference between the new school and the old is really the difference between clear and misty conceptions.

Our modern science is a philosophical system, based on ideas distinctly stated and consistently developed. The chemists of the old school can hardly be said to have had a philosophy, but they had an admirable nomenclature, which was almost as good as a philosophy, and served to classify the facts while the fundamental