IN THE LAST HALF-CENTURY. 47 



in the eighteenth century. It means that 

 the same mass of a body, under the same 

 circumstances, always requires the same 

 quantity of heat to raise it to a given 

 temperature, but that equal masses of 

 different bodies require different quanti- 

 ties. Ultimately, it was found that the 

 quantities of heat required to raise equal 

 masses of the more perfect gases, through 

 equal ranges of temperature, were in- 

 versely proportional to their combining 

 weights. Thus a definite relation was es- 

 tablished between the hypothetical units 

 and heat. The phenomena of electrolytic 

 decomposition showed that there was a 

 like close relation between these units and 

 electricity. The quantity of electricity 

 generated by the combination of any two 

 units is sufficient to separate any other 

 two which are susceptible of such decom- 

 position. The phenomena of isomorph- 

 ism showed a relation between the units 

 and crystalline forms ; certain units are 



