IN THE LAST HALF-CENTURY. 53 



that singular conception of 'bonds' in- 

 vented to colligate the facts of 'valency' 

 or 'atomicity,' the first of which took 

 some time to make its way ; while the 

 second fell into oblivion, for many years 

 after it was propounded, for lack of em- 

 pirical justification. As for the third, it 

 may be doubted if anyone regards it as 

 more than a temporary contrivance. 



But some of these hypotheses have 

 done yet further service. Combining 

 them with the mechanical theory of heat 

 and the doctrine of the conservation of 

 energy, which are also products of our 

 time, physicists have arrived at an entire- 

 ly new conception of the nature of gaseous 

 bodies and of the relation of the physico- 

 chemical units of matter to the different 

 forms of energy. The conduct of gases 

 under varying pressure and temperature, 

 their diffusibility, their relation to radiant 

 heat and to light, the evolution of heat 

 when bodies combine, the absorption of 



