BIRTH AND AFFINITIES OF CRYSTALS 125 



the supersolubility curves for triphenylmethane in organic 

 liquids, and have shown how they explain the course of 

 crystallisation even when that substance forms crystalline 

 compounds with the solvent. These authors suggest that the 

 metastable condition is to be explained by the increased solubility 

 which is known to belong to very small crystals. The first 

 products of spontaneous crystallisation must accordingly be 

 more soluble than the larger crystals for which the solubility 

 curve is determined. They cannot therefore persist through 

 the early stages of growth until the degree of supersaturation 

 becomes sufficient to counteract the solubility. 



Many peculiarities or apparent anomalies in the crystallisa- 

 tion of solutions can be explained when account is taken of 

 their passage from the metastable to the labile condition. 



An experiment familiar to every schoolboy is the scratching 

 of the inside of a test tube containing a strong solution with a 

 glass rod, and the appearance of the first crystals along the line 

 of scratch. This is often explained as due to a tendency of 

 crystals to grow on rough surfaces and protuberances rather 

 than on a flat, smooth surface. 



If this explanation were correct, it ought to make no 

 difference whether the scratch be made before or after the tube 

 is filled with the solution, and the crystals ought to appear 

 along the lines of old scratches. In reality, it is necessary 

 that the scratch should be made within the liquid; moreover, 

 we have found that no effect is produced until the solution has 

 been supersaturated to the exact point at which it is becoming 

 labile. The process of scratching merely supplies the mechanical 

 disturbance sufficient to make the crystals grow in the labile 

 solution. 



It is true that scratching may produce a line of crystals in a 

 metastable solution ; but this only occurs if the rod is intro- 

 duced from outside, and has caught up crystalline germs in 

 the air or at the surface of the liquid ; no effect is produced 

 by scratching with the end of a rod which has been kept in 

 the metastable liquid throughout the cooling. 



Now a strong aqueous solution, being merely a mixture of 

 the dissolved substance and of ice, both in the liquid condition, 

 a similar supersolubility curve ought to be obtained for the 

 spontaneous freezing of ice in solutions which contain excess 

 of water. We have not conducted any experiments at sufficiently 



