on Isomorphism. 411 



upon the evidence of these analyses, that potash and soda are 

 capable of replacing each other in equivalent proportions ; but 

 I admit there is abundant evidence to prove that they replace 

 each other in proportions which are quite indefinite. 



The only results which are at all favourable to Professor 

 Johnston's position, are those of Vauquelin and Lehunt; in 

 these I admit that the potash is to the soda nearly as 3 to 2, 

 which are proportional to their atomic weights ; but in one of 

 Lehunt's analyses the quantity of lime is more than twice as 

 great as that in Vauquelin's. 



According to Berzelius the potash in one specimen amounts 

 to 1*7, and the soda in the other to 12*19 ; according there- 

 fore to the rule which I have admitted, the soda should be 

 only frds the weight of the potash, or 1*12 instead of 12*19; 

 so that here we have one equivalent of potash isomorphized 

 by about 10 equivalents of soda. This is quite at variance 

 with the rule which I have quoted from Professor Johnston, 

 viz. " that binary compounds which replace each other, con- 

 tain not only the same absolute number of atoms, but also the 

 atoms of the two elements in the same relative proportion." 



After alluding to an anomaly formerly presented by the alums, 

 but now supposed to be explained, respecting the isomorphism 

 of potash and soda. Professor Johnston continues : "But all 

 doubt has at length been removed from the relation between 

 the forms of potash and soda by a beautiful observation of 

 Frankenheim (Pogg., xi. p. 447). He has found that when a 

 saturated solution of pure nitrate of potash is left in small 

 quantity to spontaneous evaporation, two sets of crystals are 

 formed, one in prisms, the other in rhomboids ; the former the 

 common arragonitic form generally assumed by nitrate of 

 potash, the latter that of calc spar, commonly assumed by 

 soda. The rhomboidal crystals are microscopic and pass into 

 the prismatic form by friction, by pressure, or by contact with 

 a prismatic crystal, and hence when the salt is crystallized in 

 large masses they entirely disappear. It may therefore be 

 considered as demonstrated that the nitrates of potash and 

 soda are at once isomorphous and dimorphous, or isodimor- 

 phous ; and since the potash and soda replace each other in 

 certain mineral compounds (chabasie for example) the alkalies 

 also, perhaps the metallic radicals themselves, may be consi- 

 dered isodimorphous." 



I know not whether in the original paper cited by Professor 

 Johnston it is stated that the " microscopic" crystals were sub- 

 mitted to measurement; if not I confess that I am not ready 

 to admit the case as proved, for it might be easy to confound 

 " microscopic" rhomboids and shortened prismatic crystals. 



Professor Johnston, in alluding to the researches of Dulong 



