1886.] on Suspended Crystallisation. 509 



as it is dissolved the water will take up nearly twice its weight of 

 salt. If this solution be now allowed to cool in an open vessel an 

 abundant deposition of crystals will take place, as the water when 

 cold will only dissolve about one-third of its weight of crystallised 

 sulphate. But if the flask be tightly corked or stoppered with cotton 

 wool whilst the solution is boiling, it may be kept for several days 

 without crystallising, although moved about from place to place. On 

 withdrawing the plug, however, the air on entering the flask will pro- 

 duce a slight disturbance of the surface of the fluid, and from that 

 point beautiful prismatic crystals shoot through the solution until the 

 whole has become perfectly solid. 



It is not necessary in preparing such solutions that the flask be 

 closed completely with a stopper, as we find that plugging the neck 

 with cotton wool exercises the same eflfect in preserving the solution, 

 and that the air which enters the flask during the cooling becomes 

 thoroughly deprived of its crystallising influence, evidently under- 

 going a filtering process. A very good instance of the inability of 

 filtered air to start crystallisation is afforded by a solution of alum 

 saturated at 194"^ F., and allowed to cool in a flask stoppered with 

 cotton wool in the manner previously described. On withdrawing 

 the plug of cotton wool the crystallisation, which in this case takes a 

 much longer time to commence than with the sulphate of soda, will 

 be seen beginning at various points on the surface of the liquid, and 

 will spread slowly from these, octahedral crystals of alum half an inch 

 or more in length being built up in a few seconds. 



It is evident from these two experiments that the cause of this 

 sudden crystallisation is to be sought for in the peculiar action of the 

 air when it comes in contact with the solution ; but that it is not due 

 to the action of the air alone is shown by the fact that the air in the 

 flask plugged with cotton wool must enter during the cooling, and 

 that therefore the action of the cotton by filtering the air in some 

 way deprives it of its active property. 



But it will be found that there are other means of destroying this 

 activity without filtering the air through cotton wool. Thus, if the 

 solution of sodium sulphate containing two-thirds of its weight of 

 crystallised salt be allowed to cool in a flask closed by a cork, fur- 

 nished with two tubes plugged with cotton wool ; on removal of the 

 cotton plugs air may be blown from the lungs through the longer 

 tube without causing the crystallisation, apparently from the fact that 

 the air has been deprived of the nucleus which induces the crystallisation 

 by passage through the lungs. On blowing air, however, from a 

 bellows, after a few strokes the solution will be found to solidify 

 almost instantaneously. 



The earliest ideas concerning this sudden solidification were 

 naturally those which supposed that the mere entrance of air into the 

 flask upon opening it started the crystallisation. The late Dr. Graham, 

 and Professor Thomas Thomson, held this view ; and the former of 

 these chemists carried out a considerable series of investigations on the 



