A HUNDRED YEARS OF CHEMISTRY. 61 



liberated or absorbed — a distinct, measurable quantity. This fact 

 was established by Hess in 1840, and since then the thermal values 

 of many reactions have been determined, notably by Thomsen in 

 Denmark and Berthelot in France. The data are already numer- 

 ous, but as yet they have not been co-ordinated into any general 

 law. They are in great measure the raw material with which 

 some future scholar is to build. One fact, however, is already 

 clear — namely, that the heat of formation or of combustion of 

 any compound is conditioned by its structure. Two isomeric sub- 

 stances may differ widely in their calorific constants, an observa- 

 tion which has repeatedly been verified. Thus the conception of 

 structure, of atomic grouping, appears again a chief factor in a set 

 of unsolved problems. 



In the relations of chemistry to heat perhaps the greatest ad- 

 vances have been made in the extension of our resources, particu- 

 larly in regard to the development and control of temperatures. At 

 the beginning of the century the range of temperatures available 

 to the chemist was narrowly limited — from the freezing point of 

 mercury at one end to the heat of a blast furnace at the other. 

 His command of heat and cold are now vastly greater than then, 

 and the steps which have been taken are worth tracing. 



At the lower end of the scale the greatest progress has been 

 made through the liquefaction of gases. When a liquid evapo- 

 rates, heat is absorbed, or, reversely stated, cold is produced, and 

 the more rapid the evaporation the greater is the cooling effect. 

 A command of more volatile liquids is therefore a command of 

 cold, and the liquefied gases represent the extreme limit of our 

 power in that direction. 



Near the beginning of the century, by combining cold and pres- 

 sure, sulphurous acid and chlorine were reduced to the liquid 

 state. In 1823 Faraday succeeded in liquefying still other gases, 

 and in 1835 Thilorier went even further and reduced carbonic acid 

 to a snowlike solid. Liquid chlorine, sulphurous acid, and carbonic 

 acid, stored in strong cylinders of steel, are now commercial prod- 

 ucts, manufactured and sold in large quantities like any other 

 merchandise. They can be transported to long distances and kept 

 indefinitely, to the great convenience of chemists and the further- 

 ance of research. 



In 1845 Faraday published the results of further investigations, 

 when it appeared that all but six of the known gases had been re- 

 duced to the liquid state. Through cold and pressure lower and 

 lower temperatures were gained, each step forward having given 

 a foothold from which a new advance was possible. In 1877 Pictet 

 and Cailletet simultaneously succeeded in liquefying four of the 



