Reports and Proceedings — British Association. 521 



of growth or nuclei which appear in a supersaturated solution, does 

 not recognise any distinction between the two states. 



During the present year a number of experiments carried on by 

 Miss F. Isaac and myself upon the strength of solutions from which 

 crystals are growing have shown that it is easy to determine the 

 changing concentration of a cooling solution by an optical method, 

 to show that it passes into the labile state, and to ascertain the 

 temperature at which the transition occurs. We have found, 

 for example, that a solution containing 48 per cent, of NaNOg is 

 saturated at 26°, is metastable between 26° and 16°, and crystallises 

 spontaneously below that temperature ; one containing 52 per cent, 

 of Na N O3 is saturated at 44°, and becomes labile at 35°. 



In the metastable state inoculation by a solid germ of the dissolved 

 substance, or of one isoraorphous with it, is necessary in order 

 to cause the liquid to crystallise ; in the labile condition solid germs 

 may be spontaneously generated from the liquid. Take, for example, 

 a test tube filled with a solution of sodium-nitrate containing 48 

 parts of the salt in 100 parts of solution, which is metastable at 

 ordinary temperatures : if crystals make their appearance in this 

 solution it will only be because the dust of the room contains minute 

 particles of sodium-nitrate which fall into the tube, or because 

 crystals are deposited where drops have evaporated near the surface, 

 and accordingly the first crystals appear at the surface of the liquid, 

 and grow there until they are large enough to fall to the bottom. 

 I find that such a solution, if enclosed in a sealed tube so as to 

 prevent access of germs and evaporation, cannot be made to 

 crystallise above the temperature of 16°, although it is supersaturated 

 at all temperatures below 26°. 



Again, let a hot solution of the same strength containing 48 per 

 cent, of the salt be allowed to cool down while being stirred. If 

 dust containing Na N O3 can be excluded, the liquid will not 

 crystallise until the temperature falls to 16°, when the solution 

 passes from the metastable to the labile condition. A cloud of nuclei 

 will then form throughout the liquid, and each will proceed to grow 

 as a separate crystal ; the immediate effect is to reduce the liquid to 

 the metastable state so that no more crystals are produced, but each 

 of these continues to grow from the liquid with which it is in contact. 



If dust be not excluded, crystals may make their appearance upon 

 the surface of the liquid and will soon sink ; but even though they 

 be stirred about actively in the solution the liquid as a whole 

 remains in the metastable state till a temperature somewhat below 

 16° is reached, when the labile region is entered and a cloud of new 

 crystals makes its appearance. 



It follows, therefore, that in a cooling supersaturated solution, 

 from which germs have not been excluded, there are normally two 

 periods of growth : one in which a comparatively small number of 

 isolated crystals are growing regularly, and a subsequent period 

 in which a shower of small crj'stals is produced. Only if the rate of 

 cooling be sufficiently slow, or the stirring be sufficiently violent, to 

 keep the liquid in the metastable condition will there be no second 

 period, no sudden precipitation of nuclei. 



