CH. XXVII. RAPID ADVANCE OF CHEMISTRY.

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something into the atmosphere but take a gas out of it. Dr. Black was one of the first to be convinced, but Priestley died without giving up his old opinions. The younger chemists, however, saw the truth of Lavoisier's explanation, and from this time chemistry advanced very rapidly. Lavoisier invented an entirely new set of terms instead of the old names of the alchemists, and though his terms have been greatly altered by later discoveries, still many of them will always be used. He repeated with a better apparatus Cavendish's experiment of turning hydrogen and oxygen into water, and he gave hydrogen its name from dwp, water, and yerváw, I produce. Lastly, he published his 'Elements of Chemistry,' in which he gave a clear explanation of the different chemical changes, and how students could work them out for themselves.

Lavoisier was now at the height of his fame, full of his new theory, and prepared to devote the rest of his life to making chemistry a grand science; but a very sad fate was awaiting him. In 1793 the great French Revolution broke out in Paris. Lavoisier was a farmer-general, that is a kind of collector of taxes, and all the farmers-general were hated by the people; so he knew that he should most likely lose all his fortune, and was prepared to work for his living; but he had not expected the blow which fell upon him. All the farmers-general were condemned to death, and though a physician named Hallé, who deserves always to be remembered for this act, pleaded that Lavoisier's life should be spared till he had completed his experiments, the ignorant and brutal Government replied, 'We do not need learned men,' and on May 18, 1794, at the age of fifty-one, Lavoisier was guillotined.

After his death the French School of Chemistry took the

lead for many years, until new discoveries in England, which we shall mention by-and-by, made another great advance. When you are able to read larger works upon the history of chemistry you will find how very interesting the period was of which we have been speaking. I have only been able to give you here the very barest outline of it, so that the names of these great chemists may not be quite unfamiliar when you meet with them in other books.

Chief Works consulted.—'Three Papers on Factitious Air,' by Cavendish 'Phil. Trans.,' 1766; Brande's Chemistry;' Hoefer's 'Histoire de la Chimie ;' Cuvier, 'Histoire des Sciences Naturelles ;' Huxley, 'On Priestley'-'Macmillan's Magazine,' 1874; Priestley, ‘On Different Kinds of Air,' 1774; Thomson's Hist. of Royal Society;' Scheele's 'Chemical Experiments on Air and Fire,' translated 1780; Miller's 'Elements of Chemistry;' Lavoisier's 'Elements of Chemistry,' translated by Kerr, 1790.

CH. XXVIII.

DR. BLACK.

241

CHAPTER XXVIII.

SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED).

Doctrine of Latent Heat, taught by Dr. Black in 1760-Water containing Ice remains always at o° C., and Boiling Water at 100° C., however much Heat is added-Black showed that the lost Heat is absorbed in altering the condition of the Water-Watt's Application of the Theory of Latent Heat to the Steam-engine-Early History of Steam-engines-Newcomen's Engine-Watt invents the Separate Condenser-Diagram of Watt's Engine-Difficulties of Watt and Boulton in introducing Steam-engines.

Discovery of Latent Heat by Dr. Black in 1760.-We must now go back a few years, to the time when Dr. Black was lecturing at Glasgow in 1760; for he then made a remarkable discovery about heat, which belongs to the history of physics rather than of chemistry. This was

the discovery of latent heat, or of heat which becomes lost or hidden whenever ice is turned into water, or water into steam.

If you put a lump of ice in a saucepan on a stove, and when it begins to melt stir it gently so as to keep the water well mixed, you will find that so long as the smallest piece of ice is left in the water, a thermometer standing in the saucepan will not rise higher than o° Centigrade, or the meltingpoint of ice. Now the heat from the stove must be continually entering the water, otherwise the ice would not melt. What then becomes of this heat? Again, if you keep the water on the stove after the ice is melted, it will grow hotter

and hotter till it reaches 100° Centigrade, when it will boil. Here, again, it will remain at the same temperature, and though you go on boiling it till it has all passed away in steam, the last drop of water will never be hotter than 100° C. So that here again the heat which is added remains hidden and does not become apparent. This last fact about boiling water had been long known to philosophers, but no one found any explanation of it until Black began his experiments on melting ice; and he then came to the conclusion that the heat is employed in altering the condition of the water, hat is, in changing it, in the one case from solid ice into water, and in the other from water into a vapour.

He proved this by some simple experiments which are not difficult to make. He took two glass flasks, and filled one with ice just on the point of melting, and the other with an equal weight of ice-cold water. These he hung in a moderately warm room, which he kept all the time at the same heat (8°5 C.). At the end of half an hour the icecold water had risen four degrees (from o° to 4°), but the melting ice remained at o°, and it was ten hours and a half before the ice had disappeared and the water had reached the same temperature as that which the water in the other basin had attained in half an hour. Now the melting ice had been receiving heat for twenty-one half-hours, and therefore had taken in 21 × 4, or 84° of heat, while it only showed a rise of 4°. It was clear, therefore, that the remaining 80° must have been spent in turning the ice into water.

He

Black now tried the same thing in another way. found that a pound of water at 79° C. would exactly melt a pound of ice. So he again took two vessels, in one of which he put a pound of ice-cold water at o° and a pound of hot

CH. XXVIII.

LATENT HEAT.

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water at 79°, and when they were properly mixed he found, as he expected, that the heat of the mixture was half-way between the two, that is 391. In the other vessel he put a pound of ice at o° and a pound of hot water at 79°, and here, when the ice had disappeared, the mixture still remained at o°, showing that the whole 79° of heat in the boiling water had been absorbed in melting the ice, and remained hidden or latent in the two pounds of water. The latent heat of water is therefore between 79° and 80°.

We know now what becomes of this heat, as you will see (chapter xxxiv.) in the history of the science of the nineteenth century; but the first step was to prove its disappearance into the water, and this we owe to Black; as well as the fact that still more heat is lost in turning water into steam.

This last fact he proved by filling a glass bottle half full of water, corking it very tightly, and then heating the bottle till the water began to boil. He was obliged to do this very gently, because steam expands with great power, and he did not wish to drive out the cork or break his bottle. After a little time the water ceased boiling, because the other half of the bottle was full of steam, and there was no room for more to form. But now the water began to grow hotter and hotter, and rose above 100° C., showing that when the heat could no longer form steam it did not remain hidden, but increased the temperature of the water. At last, when he was afraid to heat the bottle any more, he loosened the cork, which flew out with great violence, followed by a cloud of steam. And now notice what happened; directly the rush of steam was over, the heat of the water in the bottle fell again to 100° C., for all the rest of the heat had been used in forming more steam the moment the pressure was removed.