﻿ATOMIC HEAT TO CRYSTALLINE FORM. 123 



form of arragonite. Now the latter carbonates have a somewhat higher atomic heat 

 than SrO, C0 2 . 



If hot, concentrated caustic potash be saturated with protoxide of lead, the latter 

 separates in the shape of rhombic octohedrons having a yellow color; as the 

 solution cools, however, red, crystalline scales are deposited on the first. If the red 

 crystals be heated, they take the first form and also the yellow color.* 



Anatase and rutile, two forms of oxide of titanium, though both belonging to the 

 square prismatic system, have incompatible angles, which assign to each a different 

 primitive form. They differ in atomic heat and volume. 



Bisulphuret of iron, as common iron-pyrites, belongs to the regular system ; as 

 white iron-pyrites, to the right prismatic system, — the latter being of a paler yellow 

 color and much softer. Breithaupt thinks that the former has been formed at a 

 higher temperature than the latter. The atomic heat of the common variety is 

 7.5480; of the white variety, 7.8854. 



Sulphate of nickel (NiO, S0 3 7Aq.) is trimorphous. It crystallizes below 59° F. in 

 right rhombic prisms ; between 59° and 68° F. in acute octohedrons with square bases ; 

 and above 86° F. in oblique rhombic prisms; also in forms belonging to the right, 

 square and oblique prismatic systems. The right rhombic prisms, after exposure to 

 sunlight for a few days, are found to be composed of square-based octohedrons, often 

 several lines in length. The sulphates of zinc and magnesia exhibit similar pheno- 

 mena. They assume different forms, according as they are made to crystallize at a 

 high or a low temperature,—- manifesting, as Mitscherlich has shown, very peculiar 

 and interesting changes with variation of temperature.^ 



According to Frankenheim, sal ammoniac, which generally crystallizes in regular 

 octohedrons, appears at higher temperatures to assume forms belonging to the right 

 prismatic system. J 



When the rhombohedrons of KO, N0 5 are heated considerably above 100° C, they 

 change into prismatic crystals. A hot solution of KO, N0 5 deposits prismatic crystals 

 when slightly cooled; at a temperature of 14° F., however, rhombic crystals begin to 

 appear. (Frankenheim.) 



Protiodide (red iodide) of mercury sublimes at a low heat, and also separates from 

 solution in square, prismatic tables of a scarlet hue ; at a higher temperature, rhombic 

 tables of a sulphur-yellow color are formed. As often as they are heated, the red 

 crystals turn yellow, and again become red as they cool. (Gmelin.) 



Examples of this kind attest the strong, controlling influence of temperature upon 

 isomorphism. We see that whether a body shall be isomorphous with a second or o 

 third, is mainly determined by temperature. "Les isomorphismes chimiques d'un 



* Mitscherlich. Jour Pr. Cbem. 19, 451. j Gmelin. J Jour. Pr. Chtm. 16, 3. 



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