PROGRESS IN PHYSICAL CHEMISTRY. 293 



alloy, which, as regards its density, gives the largest 

 exception from the admixture rule, has also the lowest 

 melting-point, and has the approximate composition Pb 

 Sn 3 . This result differs from those of previous observers. 

 Paterno and Montemartini (84) also assert that certain 

 eutectic mixtures have a definite chemical composition. 

 They find, for example, that mixtures containing one 

 molecular proportion of paraxylene to two of acetic acid, or 

 one of paraxylene to three of trimethyl-carbinol, are eutectic, 

 and they conclude that these results throw light on the 

 existence of molecular aggregates in solution. It has to be 

 stated, however, that the existence of eutectic mixtures can 

 readily be explained without the assumption of chemical 

 combination. 



A curious point regarding the structure of isomorphous 

 mixed crystals has been noticed by Ambron and Le Blanc 

 (85). Mixed crystals of lead and barium nitrates were 

 doubly-refracting. On treating them with a saturated 

 solution of barium nitrate the lead nitrate was dissolved out, 

 but a skeleton remained, which consisted of regularly 

 arranged particles of barium nitrate of about 0*03 mm. 

 side, and the double refraction had disappeared. From 

 these and other experiments it appears that the constituents 

 of a mixed crystal are in a state of rough, mechanical 

 mixture, and not in a state of molecular admixture as has 

 been commonly supposed. 



Muthmann and Kuntze (86) have determined the 

 solubility in water of three pairs of isomorphous salts, 

 namely, mono-potassium phosphate and arseniate, potassium 

 chlorate and permangate, and potassium and rubidium per- 

 manganates. The curves showing how the amount of one 

 constituent present in the solution varies with its amount in 

 the mixed crystal are very different in each case, the note- 

 worthy point being that they correspond with Roozeboom's 

 predictions (87), based upon the analogy between the 

 solubility of mixed crystals and the evaporation of mixed 

 liquids. 



