PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 623 



Tlie absolute field of color attainable by the prism is dependent upon the 

 intensity of the light. 



Testing Oils by their Cohesive Figures. 



Chemistry having failed to detect some admixtures of oils, Mr. C. Tom- 

 linson has had recourse to a physical test, doubtless suggested by the 

 researches of Savart, which he described before the London Pharmaceuti- 

 cal meeting of last month. The test depended upon the forces of cohesion, 

 adhesion and diffusion. A drop of the liquid to be tested was carefully 

 dropped upon chemically pure water in a chemically clean vessel. The 

 force of adhesion endeavored to spread the oil in a thin film over the sur- 

 face of the water, the force of cohesion endeavored to prevent this, and 

 from the contest of these two forces there resulted figures which the author 

 believed to differ in the case of every independent liquid. There might be 

 instances of two or more liquids differing in chemical composition but hav- 

 ing the same physical properties, where the figures would be identical* 

 Such cases, however, must be rare. The oil must be dropped from a rod that 

 has been made perfectly clean by being washed with strong sulphuric acid, 

 rinsed in water, then washed with caustic potash, and again rinsed with 

 water. The rod should be held only about one-tenth of an inch from the 

 water when the oil is dropped. 



The figure produced by castor oil is of great beauty. As soon as this 

 oil is dropped, it spreads out in a series of rings, magnificently irridescent. 

 Outside there is a silver corona, which breaks up into a delicate, lace-like 

 pattern; this appearance lasted for some seconds. The figures of the fixed 

 oils are more durable. Mixtures could be easily detected, as the figures 

 produced partook of the characters of both the constituent oils, yet resem- 

 bling the proper figure of the oil which is in excess. A mixture of lard oil 

 with castor oil is best shown in what may be called the residual phenomena, 

 the lace-like patterns being scattered over with numerous small blotches. 

 Croton oil gives a magnificent figure of large pattern, and a mixture of five 

 per cent, of this oil with castor oil may be readily detected by the practiced 

 eye. Turpentine gives a remarkable figure; the film flashes out immedi- 

 ately almost to the edge of the vessel; the outer circumference is dotted 

 with numerous small bosses, which are larger within, and enclose patches 

 of irridescent colors. The figure produced by balsam copaiba was a mag- 

 nificent sight. A succession of rings were thrown out showing beautiful 

 colors and a metallic luster. A mixture of balsam and castor oil gives 

 neither colors nor pattern. In conclusion Mr. Tomlinson said he did not 

 look upon the investigation as complete — he believed the figure of every 

 pure liquid would serve for its verification, as the form of a crystal led to 

 the identification of a salt. 



New Facts in Fermentation. 



M. A. Becharaps states in the Comptes Rendus that if the must of grapes 

 of different kinds is filtered, the ordinary alcoholic ferment (yeast) only 

 appears on it; but if not filtered, thread-shaped ferments make their appear- 

 ance also, and occur largely with the free access of air. The filtered must 

 yields a wine that differs considerably from that of the unfiltered grape 

 juice made in the ordinary way. 



