242 Mr. C. Toinlinson on some Effects of 



The reason why this crystalline crust does not act as a nucleus 

 to the rest of the solution is, that, being chemically clean, there 

 is perfect adhesion between it and the solution as a whole, and 

 hence there can be no separation of the salt from the water of the 

 solution. 



An objection to this theory has been put : — In ordinary crys- 

 tallization, as in nursing a crystal of alum, are we not dealing 

 with chemically clean surfaces ? 



My answer is a decided negative. The evaporating-dish that 

 contains the solution is not chemically clean ; the solution itself, 

 exposed as it is to the air, is not chemically clean ; nor is the 

 hair by which the crystal is suspended ; nor is the crystal itself, 

 for this is frequently taken out and exposed to the air and 

 handled, and abnormal growths chipped oiF with the thumb- 

 nail. Under such conditions of chemical impurity the crystal 

 acts as a powerful nucleus ; and if the fresh crystalline deposits 

 made upon it are chemically clean, they do not long remain so. 

 A chemically clean octahedron of alum is transparent ; a nursed 

 crystal is opake, (as I believe) in consequence of the multitude 

 of minute crystals formed upon the impurities of the surfaces. 



The general opinion is that the most powerful nucleus for 

 the crystallization of a saline solution is a crystal of the salt 

 itself. It is nevertheless possible, with strict attention to che- 

 mical purity, to introduce a crystal into a highly supersaturated 

 solution of the same salt, and yet the crystal shall not act as a 

 nucleus, the solution remaining perfectly liquid as before. 



Eor this purpose a solution of two parts by weight of mag- 

 nesic sulphate and one of water was boiled and filtered into a flask 

 made clean by means of strong sulphuric acid, and rinsing with 

 water; the solution was again boiled in the second flask; and 

 while steam was issuing from the neck, a short tube, suspended 

 by a wire, full of crystals of magnesic sulphate (all previously 

 dipped in sulphuric acid and rinsed with water), was held in the 

 neck, and secured there by putting in the plug of cotton-wool at 

 the same time that the spirit-lamp was removed. The flask was 

 left to cool during about fifteen hours, and then, without dis- 

 turbing the cotton-wool, the tube with the crystals was lowered 

 into the solution : there was no crystallization ; and during forty 

 hours, though the flask was shaken several times, the solution 

 remained unchanged. 



A similar solution was filtered into clean tubes, which were 

 closed with cotton-wool, and when nearly cold were put into 

 strong sulphuric acid, covered with a receiver, and the air ex- 

 hausted. In the course of twenty minutes a crystalline crust 

 formed on the surface ; and this, during the shaking of the pump, 

 fell to the bottom of the solution ; but it did not induce crystal- 



