WHY IS H2O THE SYMBOL OF WATER?

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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

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principles of the science were being slowly developed. It was, of course, to be expected that the fundamental ideas of our science should be conceived separately and at first only imperfectly; and it was not until clear and definite conceptions had been reached, and the relations of the several ideas clearly understood, that a philosophy of chemistry was possible. Of course, we are far from believing that the ideas, now prevailing, are necessarily true, and it is perhaps to be expected that our modern school will share the same fate as that which preceded it; but we do believe that the coming system, whatever it may be, will be based on equally clear conceptions, and that, in attempting to clarify our ideas and realize our conceptions, we are following the right path, and making the only satisfactory progress.

Before closing the lecture, it only remains for me to show how the system of notation I have described may be used to express chemical changes, and I can best illustrate this use by applying it in a practical example. The experiment I have selected for the purpose must be familiar to every one in some form or other.

In the first place, we have in this large glass vessel a white, pulverulent solid, familiarly called soda. The chemists call it sodic carbonate. It consists of molecules, which are each formed of six atoms, two of a metal called sodium, one of carbon, and three of oxygen. Hence, the symbol is Na2CO3. In the second place, we have in this pitcher a liquid well known in commerce under the name of muriatic acid. It is a solution in water of a compound which is called in chemistry hydrochloric acid. Hydrochloric acid itself, as I shall show you at the next lecture, is a gas 181 times as heavy as hydrogen; hence its molecular weight is 36-and its molecules, as is well known, con

sist of one atom of chlorine and one of hydrogen. Its symbol is then HCl—and the condition of aqueous solution we may express by the addition of the letters Aq, the initial of aqua, the Latin name of water-thus: HCl + Aq.

On pouring the acid upon the soda, there is at once a violent effervescence; and a large quantity of gas is evolved, which will soon fill the glass jar. The old substances disappear, and new substances are formed. This, then, is a chemical process, and such a process, in the technical language of chemistry, is usually called a reaction; and as hitherto we have spoken of the factors and products of a chemical process, so hereafter we shall use the same terms in describing chemical reactions.

In the present example, the factors are sodic carbonate, hydrochloric acid, and water. What are the products?

First of all, we have a large volume of colorless gas, and not only a large volume, but also a very considerable weight, since, for a gas, it is quite a heavy substance. In old times this product of the process was wholly overlooked; but I can easily prove to you that there is a no inconsiderable amount of material in the upper part of this glass vessel, although in an invisible condition. First, by lowering a lighted candle into the jar, I can show that the air has been displaced by a medium in which the candle will not burn. In the second place, by dipping out some of the gas and pouring it into this paper bucket, I can make evident that its weight is appreciable: You notice that the end of the balance-beam to which the bucket is suspended. immediately falls; and see, also, how these candles are extinguished, as the heavy gas from my dipper flows down on the flames. Lastly, by repeating the experi

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ment on a smaller scale in front of the lantern, and projecting the image of the small glass vessel, we here use, on the screen, I can make the current of gas visible as it flows over the lip.

This aëriform material is now called in chemistry carbonic dioxide, but you are more familiar with it under the old name of carbonic acid. It is the chief product of the burning of coal and wood; and, when you are told that every ton of coal burned yields 3 tons of this gas, you can conceive what immense floods are being constantly poured into the atmosphere from the throats of our chimneys. It is also being continually formed, and in still greater amounts, by the processes of respiration, fermentation, and decay. Although familiarly known only in the state of gas, it can readily be reduced by pressure and cold to the liquid condition; and, when in this condition, is easily frozen, forming a transparent solid like ice, or a loose, flocculent material like snow, under different conditions. It is a compound simply of carbon and oxygen, and no fact of chemistry is better established than that every molecule of this gas consists of one atom of carbon and two atoms of oxygen. Hence its symbol is CO2.

The presence of the other products formed in our experiment I cannot make so readily evident to you, although they are really far more tangible than this gas. One of them is water, which at once mingles with the large body of water used in the experiment. The other is common salt. This dissolves, as it forms, in the water present; but, after the reaction is ended, it can easily be isolated by evaporating the brine. We will start the process, so that any one who is skeptical can satisfy himself, by tasting the residue, that common salt has been really formed.