QUANTITY OF HEAT. SPECIFIC HEAT. 



87 



Neuman first pointed out that in certain compounds of similar con- 

 stitution the product, Molecular weight x specific heat, or " molecular 

 heat," is nearly constant. Regnault investigated the molecular heats 

 of a very large number of constants, and found that each class had its 

 own molecular heat. The results for the members of a class differ, 

 however, several per cent, from the mean. 



Later, Kopp took up the investigation, and found that by assigning 

 to each atom its own atomic heat not the same for all elements a very 

 great number of quite different compounds in the solid state come under 

 the following rule (known as Kopp's Law) : " The molecular heat of a solid 

 compound is the sum of the atomic heats of the constituents." 



The following are the atomic heats assigned : * 



C ... 1-8 Be ... 3-7 P ... 5-4 



H ... 2-3 Si ... 3-8 S ... 5-4 



B ... 2-7 O ... 4-0 Ge ... 5-5 



and for other elements, 6"4. 



Thus for ice the molecular heat is, by Kopp's Law, 



2x2-3 + 4 = 8-6 

 whereas experiment gives 0*474 x 18 = 8*5. 



For calcium carbonate, CaCO 3 , the calculated molecular heat is 



while experiment gives 0'203 x 99-9 = 20'3. 



For sodium chloride, Nad, the calculated molecular heat is 



6-4 + 6-4 = 12-8 

 while experiment gives -214 x 58*4= 12-5 



* Nernst, Theoretical Chemistry, p. 154. 



