298 Mr. C. Tomlinson on the Action of Low Temperatures 



deposit of crystals in the flask ; but on gently shaking it the 

 clear transparent solution immediately became opaque from the 

 multitude of octahedral crystals that at once started into exist- 

 ence. These quickly subsided ; and on the deposit a fine crop of 

 the seven-atom salt soon began to form, consisting of flat plates 

 or prisms with oblique summits from which heat-currents as- 

 cended, while the liquor above, consisting of a solution of the 

 anhydrous salt, became quite clear. 



It is perhaps useless to speculate on the condition of this so- 

 lution, reduced, as it was, perhaps 30° below the point at which 

 it usually deposits anhydrous crystals. Did they start into ex- 

 istence in a moment ? or were they present in the solution in 

 some molecular condition in which they were not visible because 

 they had the same index of refraction as the solution, until by 

 shaking they threw off, as it were, the water external to the 

 molecular groups and assumed an independent existence and 

 their own peculiar refractive density and so became visible ? 

 When water is cooled many degrees below its freezing-point, is 

 it a supersaturated solution of ice ? and is the ice really present, 

 but not visible, from the same cause ? But the salt, it may be 

 said, cannot be present and invisible, or it would fall by its 

 weight. But the viscosity of the solution may prevent this ; and 

 is there not a condition of close adhesion within the body of a 

 liquid, less than that required for solution, greater than that re- 

 quired for the independent existence of a crystal, by which a 

 heavier solid is suspended and is not visible until, by some slight 

 mechanical disturbance, that close adhesion becomes less, and 

 the solid then assumes a more independent existence and sub- 

 sides by its weight ? 



The double salt salt formed by mixing the sulphates of zinc 

 and magnesia in atomic proportions forms with boiling water 

 a solution which, when filtered into a clean tube and reduced 

 to about 20° F., behaves very similarly to supersaturated 

 solutions of Glauber's salt at higher temperatures ; that is, the 

 double salt forms at the bottom of the tube acicular crystals of 

 a lower degree of hydration, leaving the solution above still su- 

 persaturated with the double salt. On removing the cotton- 

 wool, crystallization of the double salt sets in from the surface ; 

 and when this salt reaches the mass at the bottom, the latter 

 becomes of an opaque white from splitting up into minute crys- 

 tals and appropriating an additional quantity of water of crys- 

 tallization. If before the solution crystallizes it be poured off 

 and the modified crystals be pressed between folds of filtering- 

 paper, they immediately begin to get warm from the fixation of 

 water, and their constitution becomes changed. 



This modified salt of zinc and magnesia, like the seven-atom 



