234 Dr. H. Bence Jones on the Decomposition of 



and salicine. Oils of turpentine and laurel also give mirrors, 

 but with still greater difficulty, the solutions requiring to be 

 very concentrated. Resin of guaiacum acts in a similar 

 manner. 



Oil of pimento, as is well known, consists of two oils, one 

 an acid oil, which is heavier than water, and forms crystal- 

 line compounds with the bases ; this in the course of a few 

 minutes, even in the cold, produces as brilliant a coating of 

 silver as the mixture of the oils of cassia and cloves. The 

 neutral portion of the pimento oil, which is lighter than water, 

 does not reduce nitrate of silver even after long boiling. I 

 could not succeed in forming metallic mirrors with cinnamic, 

 benzoic, meconic, komenic, tannic or pyrogallic acids, with 

 gum benzoin, eiemi or olibanum, with oil of rhodium or with 

 glycerine. 



Ingenious as Mr. Drayton's patent process certainly is, it 

 labours under a very serious inconvenience, which I greatly 

 fear will not be easily remedied. In the course of a few weeks 

 the surface of the mirrors formed by his process become dotted 

 over with small brownish-red spots, which greatly injure their 

 appearance. The cause of the spots seems to be this — that 

 the metallic silver while being deposited on the surface of the 

 glass carries down with it mechanically small quantities of a 

 resinous matter, resulting, most probably, from the oxidation 

 of the oil. This resinous matter, which is interposed between 

 the glass and the silver, in the course of time begins to act on 

 the metallic surface with which it is in contact, and to produce 

 the small brown spots already mentioned. If an excess of the 

 essential oils is employed to precipitate the silver, the metallic 

 mirror is much darker, and gets sooner discoloured than usual. 

 No doubt the alcohol present in the solution keeps up much 

 of the resinous matter ; still a little of it is almost always de- 

 posited on the silvered surface, and acts in the injurious way 

 described. 



XXXIV. On the Decomposition of Salts of Ammonia at ordi- 

 nary Temperatures. By H. Bence Jones, M.D.* 

 IT frequently happens that urine is met with which has be- 

 come alkaline in consequence of the urea passing into 

 carbonate of ammonia. Such urine, though at first showing an 

 alkaline reaction on red or very slightly blue litnuis paper, was 

 observed, as it dried \x\ the air, to have a well-marked acid re- 

 action. By adding a slight excess of ammonia to healthy urine, 



* Communicated by the Chemical Society j having been read December 

 2, 1844. 



