1879.] Influence of Colloids upon Crystalline Form, §-c. 



245 



The whole phenomenon snggests at first sight that a strong crystal- 

 line solntion can act on another crystalloid deposited within it, jnst as 

 a colloid would act. Bnt the microscopic examination demonstrates 

 that the "orate is deposited at first in the form of a colloid, and in its 

 subsequent metamorphoses, which are gradual and distributed, acts in 

 one part as a colloid, in another as a crystalloid. Uric acid is known 

 to be thrown down by acids in what has been called a hydrated form, 

 which is clearly also a colloidal form, and can be seen to be at first 

 structureless, though resolved speedily into crystals. Urate of soda 

 appears also to have its colloidal stage, in ordinary conditions of short 

 duration, but prolonged by the action of the strong saline solution. 

 The explanation then is, that the saline solution retarding metamor- 

 phosis allows of a long colloid stage, during which the slowly-formed 

 crystals are modified by that part of the substance which remains still 

 colloidal. 



It appears to me that by means of this sort of process the existence 

 of a coiloid (nascent) stage in the natural history of many crystalloid 

 substances of high molecule may be sought for and studied. In this 

 way it can be shown that uric acid, when deposited from solutions of 

 urea of certain strength, passes through colloid and spherical stages. 

 With solutions of less than 4 per cent, strength of urea, uric acid, if 

 deposited from the boiling solution, forms fine rectangular crystals. 

 But if solutions of 5 per cent, strength and upward be used, the 

 gelatinous deposit and the spheres are abundantly produced. When 

 I noticed this for the first time, I thought it might be urate of 

 ammonia, resulting from the decomposition of some of the urea. But 

 as it was deposited from acid solutions this was not probable. I 

 then collected a considerable quantity of the deposit, and submitted it 

 to organic analysis. My friend, Mr. John Moss, I\C.S., made several 

 determinations for me ; but they did not yield any definite results. 

 And as the deposit occurs with solutions of very decided acidity, I 

 have been inclined to give up a second hypothesis which I had formed 

 — that the spheres were urate of urea. If they be uric acid, it is note- 

 worthy that the colloid stage is shorter than that of urate of soda in 

 the saline solutions. 



1 have already alluded to Dr. Guthrie's masses of sulphate of copper 

 as illustrating on a large scale Mr. Eainey's original observations. 

 They are, however, formed but slowly. I am now able to bring before 

 the Royal Society an experiment, in which spheres of crystalloid 

 matter can be rapidly formed, and of such a size as to be readily 

 visible to the naked eye. 



Experiment 9. — If to a solution of urea of from 1020 to 1025 specific 

 gravity strong nitric acid be added in equal bulk, the mixture, at first 

 hot, deposits on cooling to 50° F, crystals of nitrate of urea in the form 

 of separate and distinct brilliant flakes, which subside at once. 



VOL. xxrs.. B 



