44 Chemical and Physical Notes 



Imagine now that KC1 and NaCl not only crystallise in 

 the same system, but are isomorphous in the restricted sense, 

 which is characterised by the formation of " mixed crystals." 

 It is then obvious that in the saturated solution, the particle 

 of NaCl when prevented from uniting with another particle of 

 NaCl to form a crystal, will simply unite with the particle 

 of KC1 which stands in the way, and will crystallise. Con- 

 sequently, if the solution of NaCl were originally not quite 

 saturated, the introduction of the KC1, by increasing the 

 amount of crystallisable material, would make it saturated. 

 Hence, if KC1 and NaCl were isomorphous and formed 

 mixed crystals, the cryohydric temperature of a mixture 

 of the two salts would be higher than that of the NaCl. 



The nitrates of barium, strontium and lead are iso- 

 morphous salts which form mixed crystals. In boiling 

 mixtures, it has been shown 1 that nitrate of strontium raises 

 the condensing temperature of steam by 6'53 C, and nitrate 



of lead raises it by 3'29 C., while the mixture - (NO 3 ) 2 



raises it by $-g8 C. ; and the quantity of condensed steam 

 required to produce the boiling saturated solution of the 

 mixture is exactly the sum of the amounts required for the 

 ingredients separately. In the case of the nitrates of strontium 

 and barium, the elevation of the condensing point is not as 

 great as with Sr(NO 3 ) 2 alone : the maximum temperature 

 does not remain constant for a minute, and the condensed 

 steam required to dissolve the mixture is about 25 per cent, 

 more than is required to dissolve the salts separately. 



Similarly in the case of freezing mixtures, the following 

 cryohydric temperatures were observed : nitrate of barium 

 -o7 C., nitrate of strontium - 575 C., and nitrate of lead 

 - 27 C. ; and the cryohydric temperatures of pairs of these 

 salts in equal molecular proportions: nitrates of strontium 

 and barium - 5^73 C., nitrates of strontium and lead 

 -5' 2 3C., and nitrates of lead and barium 2'53 C. 



The case of the isomorphism of salts which form mixed 



1 ' On Steam and Brines,' by J. Y. Buchanan, F.R.S. Trans. A>. S. E. 

 (1899), v l- xxxix. p. 547. (Below, p. 151.) 



