614 



Professor Thomson 



[May 14, 



of water. When the whole has cooled down under a stopper of 

 cotton wool a crystal of sodium thiosulphate may be introduced ; this 

 will pass through the acetate solution without solidifying it, but 

 will cause the immediate solidification of the thiosulphate solution. 



The following phenomena may be seen in experiments carried 

 out on mixtures of dissimilar salts. 



A. When the mixture consists of two salts which are not iso- 

 morj^hous. 



(1) Sudden crystallisation may take place, gradually spreading 

 through the solution on the addition of a nucleus, causing a deposi- 

 tion of the body belonging to the nucleus only. 



(2) That when sudden crystallisation takes place, causing the 

 deposition of both salts, there is a prej)onderance of the salt of the 

 same nature as the nucleus. 



(3) That the nucleus may remain growing slowly in the solution, 

 becoming increased by a deposition of the salt of the same nature as 

 the nucleus. 



B. When the mixture consists of two isomorphous salts. 



(1) Sudden crystallisation may occur, giving a deposition of both 

 salts, apparently in the proportions in which they exist in solution. 



(2) That when slow crystallisation takes place, the nucleus 

 increases by a deposition of the least soluble salt, showing that in 

 mixed supersaturated solutions a gradation of phenomena may be 

 experienced, passing from those shown in the crystallisation of a true 

 supersaturated solution to those shown in the crystallisation of an 

 ordinary saturated solution. 



Passing from the action of nuclei on supersaturated solutions of 

 mixtures of dissimilar salts, it is interesting to examine the action of 

 the different constituents on supersaturated solutions of double salts. 

 The following table gives the results of certain experiments with 

 such double salts : — 



Substance in solution. 



Nucleus added. 



Result. 



HgCl2(NH,Cl)2,8H20 

 HgBr2(NH"Br)2,3H"2b 



HgI,(KI), 



A1K(S0,)2,12H20 .. 

 NaNH,HAs04.4H20 



HgClj (prismatic) 



HgClz (depositeil from hot 

 solution). 



NH.Cl 



HgBr^ (deposited iu the 

 cold). 



HgBrj (deposited from hot 

 solution^. 



(NHJBr 



Hglj (needle-shnped crys- 

 tals). 



KI 



AU(SO,),18H20 .. .. 



K2SO4 



NaoHAsO.,12HoO .. .. 



(NH,)JIAsO„Aq2 .. .. 



Active. 



Both active and in- 

 active. 

 Inactive. 

 Active. 



Both active and in- 

 active. 

 Inactive. 

 Active. 



Inactive. 



