70 Royal Society : — 



never with the above result when sufficient care had been taken to 

 free the finger from nuclei. 



Exp. The finger was made greasy by dipping it into oil and im- 

 perfectly wiping it with a cloth ; it was then passed many times 

 through the flame of a spirit-lamp, and finally, while still far above 

 its normal temperature, inserted into a flask of supersaturated solu- 

 tion : the flask was chosen with a neck such that it could be entirely 

 closed by the thicker part of the finger. The flask was then trans- 

 ferred to a vessel of water, lowered artificially to 38° F., and there 

 kept, with the finger still in it, for several minutes, varying in dif- 

 ferent experiments from 10 to 15, 23, 25, 30, and 35 minutes ; and 

 although the finger was strongly pressed against the sides of the 

 flask, which was seen to be smeared all over, yet crystallization was 

 not set up when the solution was made to flow over the finger-marks, 

 which were plainly visible. That the solutions were not warmed by 

 the heat of the finger, and so rendered inactive, is proved by their 

 immediately solidifying on the insertion of a dirty glass rod. 



Exp. By means of the two modifications of bulb-tube already 

 described for the experiments with absolute alcohol, thin films of 

 various oils and other bodies were formed upon the surface of super- 

 saturated solutions without inducing crystallization. That is, a small 

 glass bulb was filled with the oil or other body and boiled, then 

 supported in the neck of the flask by a plug of cotton ; the supersa- 

 turated solution was then boiled and allowed to cool ; when quite cold 

 a drop of the liquid was forced out of the bulb on to the solution ; then 

 by a sudden jerk the lens or small globule thus obtained was flat- 

 tened out into a thin film, often iridescent — but without causing soli- 

 dification. 



In numerous instances the temperature of the solution was lowered 

 by means of ice-cold water, so as to increase its sensitiveness, but 

 with no different result. 



In many cases the oil or fatty body, such as olive -oil, Russian 

 tallow, citronella-oil, castor-oil, &c, was dissolved in ether and then 

 used ; this device was used for two reasons: — first, so that the greasy 

 matter might be much diluted and so spread over a large surface, 

 and then be left as a thin film on the evaporation of the ether ; and 

 second, so that a much smaller quantity of the oil might be delivered 

 at a time. Usually the oil collected into globules shortly after the 

 evaporation of the ether, but could generally be spread out into a film 

 again by imparting a sharp twist to the flask. 



Supersaturated solutions of sodic sulphate having films of oil, 

 benzole, turpentine, citronella-oil, &c. upon their surface have been 

 kept by the writer for several months together, and some even as 

 long as eighteen months : it is true that the oil &c. soon lost the 

 form of an iridescent film, but could be made to assume it at any 

 moment ; and the above lot of flasks were seldom allowed to stand 

 for a day without being made to do so, i. e. for the first three months 

 after their preparation and at greater intervals afterwards. Every 

 now and then a flask was caused to crystallize in order to ascertain 

 that the solutions had in no way lost their sensitiveness to a dirty 



