COHESION AND CRYSTALLIZATION 



87 



Six years ago I received from Australia an exceedingly 

 beautiful photograph of a thin pellicle found in a rain gauge. 

 Mv correspondent supposed that this strange figure might 

 have been produced under the influence of an electric or 

 magnetic field. I was able to assure him by return of 

 post that the figure was the result of the crystallization of 

 copper sulphate in a colloidal medium. In return I received 

 a letter verifying this fact, and saying that there were copper 

 works in the neigh- 

 bourhood, and the 

 air was filled with 

 the dust of copper 

 sulphate. 



Living beings 

 are but solutions of 

 colloids and crystal- 

 loids, and their tis- 

 sues are built up by 

 the aggregation of 

 these solutes. We 

 have already seen 

 how the forces of 

 crystallization are 



Fig. 29. — Crystallization of sodium chloride in a col- 

 loidal solution, giving a plant-like form. 



modified in colloid 

 solutions. This 

 force of crystalliza- 

 tion must play an important role in the metamorphoses of the 

 living organism, and influence their morphology. It may 

 therefore be of interest to investigate some of the numberless 

 forms of crystallization in colloidal solutions. 



Figs. 29 and 30 represent the forms produced by chloride 

 of sodium and chloride of ammonium respectively, in 

 solutions of gelatine of different degrees of concentration. 

 Their resemblance to vegetable growth is so remarkable that 

 several observers on first seeing them have called them *' Fern- 

 crystals. 11 



I should like here to recall to your notice the work of an 

 English observer, Dr. E. Montgomery of St. Thomas's 



