12 GEOLOGICAL MEMOIKS. 



which a chemically pure compound solidifies depends simply on its 

 own particular nature, and on the pressure ; whereas the temperature 

 at which a substance dissolved in some other body becomes solid 

 depends not only on its own nature, but also, and principally, on its 

 relation to that solvent. Certainly no chemist would think of sup- 

 posing that a solution would cease to be a solution when raised to a 

 heat of 200°, 300°, 400°, or even to one at which it begins to be self- 

 luminous — that is to say, is in what is called a state of igneous 

 fusion (feuerfliissig). Fox example, whilst we consider a mixture 

 of ice and crystallized chloride of calcium, which has become liquid, to 

 be a solution, it would not be accurate to affirm that a liquid mixture 

 of quartz and felspar is not, merely because it does not become liquid 

 until the temperature is that of a red heat. Neither can there be 

 any more doubt that what is true for solutions at a low temperature 

 would hold good at a higher. If, as an illustration, we consider the 

 case of a solution of ice and crystallized chloride of calcium in relation 

 to what takes place on becoming solid, we find that, when a certain 

 relative amount of the salt is present, the liquid begins to solidify at 

 a heat of — 10° C, and then at a somewhat lower ; and until the last 

 drop has become solid, crystals of more or less pure ice are deposited, 

 amongst which crystals of chloride of calcium are imbedded. By 

 successively increasing the relative amount of the salt, the tempe- 

 rature at which the solution becomes solid may be reduced to —20°, 

 — 30°, —40°, —50°, &c., and the result is the same as before. The 

 temperature at which the water and chloride of calcium become solid 

 varies therefore according to the proportions in which they are 

 mixed. It wiU thus be seen that the temperature at which the water 

 sohdifies can be reduced to more than 59° below the ordinary freezing- 

 point, and that of the chloride of calcium, which when alone in a 

 hydrated condition fuses at +26°, can be reduced to nearly 100° 

 below that. Sulphate of potash, saltpetre, (fee, may solidify from 

 their solutions at temperatiu^es which are from 600° to 800° below 

 their point of fusion. Moreover, every one knows that on the cooHng 

 of solutions, first the water and then the salt, or first the salt and 

 then the water, crystallize, according to the degree of their concen- 

 tration. Not however to multiply examples, if in a solution of 

 chloride of calcium containing water of which the point of fusion is 

 0°C., and chloride of calcium fusing at +26°, the less fusible does 

 not first become solid, it is no less inadmissible to suppose that 

 quartz and felspar would crystallize from the state of so-called igneous 

 fusion at their own respective melting-points. On the contrary, the 

 results obtained with ail sorts of solutions are in perfect accordance 

 with the observed facts, that in graphic granite, containing a large 

 proportion of felspar, the quartz has crystallized before the felspar, 

 whereas in other varieties of granite these two minerals crystallized 

 simultaneously, or the quartz after the felspar. If then, as Rose has 

 shown in his very recent most interesting and important memoir*, 

 quartz does in no case pass into the amorphous modification of 

 silica having the specific gravity 2-2 far from its poiut of fusion, and 

 ^' Poggendorff's A»nalen, 1859, cviii. 1 ; Phil. Mag. 1860, xix. 32. 



