xvi ATOMIC WEIGHTS OF THE METALS 381 



to 20, but falls to 5 in the range from 20 to - 188 and to 2 

 between -188 and -253. In the lowest range the atomic 

 heat varies very widely and is roughly proportional to the atomic 

 volume ; but at ordinary temperatures the atomic heats of all the 

 elements except boron and carbon fall between 5*2 and 67 ; 

 even these two elements give " normal " values at higher 

 temperatures. 



In the case of solid compounds, Kopp (1865) found that "each 

 element has essentially the same specific or atomic heat in com- 

 pounds as it has in the free state." 



The Law of Atomic Heats only leads to very rough values for 

 the atomic weight, but is sufficiently exact to give the valency, 

 i.e. the integral ratio by which the equivalent of an element 

 must be multiplied in order to give its atomic weight. 



C. ISOMORPHISM 



Mitscherlich, in 1819, discovered that identical crystalline 

 forms were shown by the following pairs of isomorphous salts : 



J KH 2 PO 4 1 /NaH 2 PO 4 ,H 2 O \ /(NH 4 )H 2 PO 4 \ 

 \KH 2 AsO 4 J tNaHjAsO^HjO/ \ (NH 4 )H 2 AsO 4 / ' 



The sodium phosphate also crystallised in a second form and 

 was therefore dimorphous. 



In series of isomorphous compounds the isomorphous elements 

 replace one another atom by atom ; the analysis of isomorphous 

 salts can therefore be used to determine atomic weights. Thus 

 in the phosphates and arsenates 75 parts of arsenic take the 

 place of 31 parts of phosphorus ; therefore if P =31, then 

 As = 75. 



Isomorphous salts of similar composition possess the property 

 of forming isomorphous mixtures or solid solutions, i.e. compo- 

 site crystals in which the different constituents are mixed as 

 intimately as in a liquid solution. If these mixtures can be formed 

 in any proportions the isomorphism is complete (compare water 

 and alcohol): if one substance will only tolerate a limited amount 

 of the other (compare water and ether) the isomorphism in incom- 

 plete. An example of incomplete isomorphism is found in the 

 potassium and ammonium phosphates formulated above, which 

 will only mix with one another to the extent of about 20%. 



