114 PROGRESS OF SCIENCE IN THE CENTURY. 



physical methods of weighing and measuring; the 

 balance, which is as familiar an emblem of chemistry 

 as the crucible, is rather a physical than a chemical 

 instrument. But the recognition that chemical and 

 physical properties are inter-dependent and must be 

 studied together, practically dates from Lavoisier, 

 and it has led to a remarkable series of physico- 

 chemical researches which may be said to form a 

 special department of science. Kopp was one of the 

 early workers ; Ostwald is now one of the leaders. 



Thermochemistry. — A new chapter in the history 

 of chemistry began with Lavoisier's study of com- 

 bustion and with the resulting recognition of the 

 indestructibility of matter. But Lavoisier left the 

 dynamics of combustion untouched, and another new 

 chapter dates from 1843, from Joule's measurement 

 of the mechanical equivalent of heat, and the result- 

 ing recognition of the conservation of energy.* The 

 phenomena of chemical activity assumed a new 

 aspect when it was clearly realised that chemical 

 changes involve only re-distribution, but in no case 

 any destruction of energy or power. This also im- 

 plied that chemical energy might be measured in 

 terms of the heat evolved or absorbed. 



Let us by means of a quotation from Ostwald gain 

 a clear impression of what the main business of 

 thermochemistry is. " Chemical energy is to us the 

 least known of all the various forms of energy, as 

 we can measure neither it nor any of its factors di- 

 rectly. The only means of obtaining information re- 

 garding it is to transform it into another species of 

 energy. It passes most easily and completely into 

 heat, and the branch of science which treats of the 

 measurement of chemical energy in thermal units is 

 ♦ See the Chapter on the Progress of Physics. 



