INSTANTANEOUS CRYSTALS. 



319 



and cool slowly, we have two solutions of hyposulphite of soda and acetate 

 of soda superposed on each other. A thread, at the end of which is fixed 

 a small crystal of hyposulphite of soda, is then lowered into the test-glass ; 

 the crystal passes through the solution of acetate without disturbing it, but 

 it has scarcely reached the lower solution of hyposulphite than the salt 

 crystallizes instantaneously. (See the test-glass on the left of fig. 307.) 

 We then lower into the upper solution a crystal of acetate of soda, suspended 

 from another thread. This salt then crystallizes also. (See experiment glass 

 on the right of fig. 307.) This very successful experiment is one of the 

 most remarkable belonging to the subject of instantaneous crystals. The 





Fig. 307. Experiment of instantaneous crystallization. 



successive appearance of crystals of hyposulphite of soda, which take the 

 form of large, rhomboidal prisms, terminating at the two extremities with 

 an oblique surface, and the crystals of acetate of soda, which have the 

 appearance of rhomboidal, oblique prisms, cannot fail to strike the attention 

 and excite the interest of those who are not initiated into these kinds of 

 experiments. 



Another remarkable instantaneous crystallization is that of alum. If 

 we leave standing a solution of this salt it gradually cools, at the same time 

 becoming limpid and clear. When it is perfectly cold, if we plunge into it 

 a small octahedral crystal of alum suspended from a thread, we perceive 

 that crystallization instantly commences on the surface of the small crystal ; 

 it rapidly and perceptibly increases in size, until it nearly fills the whole jar. 



