i2 4 SCIENCE PROGRESS 



liquid. At 26° the cooling liquid passes from the unsaturated 

 condition to a supersaturated one, in which crystals can 

 grow, but require to be introduced from without; at 16 it 

 passes into a condition in which crystals can be made to appear 

 in the liquid by mechanical means such as shaking. Between 

 these temperatures the solution is in what Ostwald has termed 

 the "metastable" condition; below 16° it is in what he terms 

 the " labile" condition : and accordingly we distinguish the two 

 showers, the first by the name " metastable shower " and the 

 second by the name " labile shower." 



In any laboratory the air must contain innumerable particles 

 of every material that is being employed in the laboratory, and, 

 so far as we know, immeasurably small germs are sufficient to 

 inoculate a supersaturated solution and make it crystallise ; in 

 fact, the crystallisation of an exposed solution must be an 

 extraordinarily sensitive test for the presence of such crystalline 

 germs. 



If now the exact temperature at which the dense labile 

 shower makes its appearance be determined for a series of 

 solutions of different strengths, the observations will trace on 

 the temperature-concentration diagram a new line, which we 

 call the " supersolubility curve," giving the relation between 

 temperature of spontaneous crystallisation and strength of 

 solution; this is the curve TT in fig. 1. 



We have determined this curve for various solutions such 

 as sodium nitrate and sodium chlorate. We have also, by 

 observations upon the refractive index, traced the changes of 

 concentration in a cooling solution, and we find that it is not 

 possible to obtain a clear representation of what actually occurs 

 without taking account of the supersolubility curve. It is 

 customary to think of a cooling solution as beginning to 

 crystallise so soon as it becomes supersaturated, and as 

 crystallising at such a rate that it remains just supersaturated 

 and no more. If this were so, the changes in the temperature 

 and constitution of the liquid would be represented by the 

 line xaf. In reality, unless the cooling be very slow, the order 

 of events is more faithfully represented by the line xabf\ a few 

 crystals appearing by inoculation at a and continuing to grow 

 very slowly, whereas a very rapid growth, which is generally 

 accompanied by a cloud of crystals if the liquid be shaken, 

 sets in at b. Messrs. Hartley and Garrod Thomas have traced 



