Dr. Stenhouse 07i Substances *uoJtich reduce Oxide of Silver. 233 



These decompositions only take place in the presence of water. 

 Crystallized sulphate of soda is not acted upon in its dry state 

 by chlorine, though the crystals contain water. 



Concluding from analogy that similar decompositions might 

 be produced by cyanogen, I treated a solution of tribasic and 

 common phosphate of soda with this gas: both were decom- 

 posed. The solution assumed a yellow colour, which by the 

 further action of cyanogen became much darker. Upon di- 

 stilling this liquid hydrocyanic acid passed over, and part of 

 the above-mentioned yellow brown acid was precipitated in 

 the residue in the form of a brownish substance, which pro- 

 bably contains the oxygen of the soda, for no cyanic acid was 

 formed ; but, as I have stated already, I hope to be soon able 

 to communicate further particulars regarding these interesting 

 decompositions. 



XXXIII. On some of the Substances which reduce Oxide of 

 Silver and precipitate it on Glass in the form of a Metallic 

 Mirror. By John Stenhouse, Ph.D.^ 



IT has long been known that aldehyde, when heated in a 

 tube with ammonio-nitrate of silver, reduces the oxide to 

 the metallic state, and forms a brilliant coating on the inner 

 surface of the tube. Three other substances, saccharic acid, 

 salicylic acid and pyromeconic acid, were also known to pos- 

 sess the same property, though the coatings which they yield 

 are much darker, and therefore less beautiful than those formed 

 by aldehyde. This was the state of our knowledge previous 

 to the announcement, about six months ago, of Mr. Drayton's 

 process for silvering mirrors in the cold, by means of ammo- 

 nio-nitrate of silver and an alcoholic solution of the oils of 

 cloves and cassia, f 



I find that the number of substances which, especially when 

 assisted by heat, give more or less brilliant coatings of reduced 

 silver, is much greater than has hitherto been supposed. 

 Thus grape-sugar forms a pretty brilliant mirror even in the 

 cold. When unassisted by heat the mirror is rather slowly 

 formed, requiring from six to twelve hours ; but when a slight 

 heat is applied it forms very readily in the course of a few 

 minutes ; the coating is much darker than that produced either 

 by aldehyde or by Drayton's process. Cane-sugar also yields 

 a mirror when assisted by heat, but none in the cold. Gum- 

 arabic and starch also yield dark-coloured mirrors, but more 

 slowly, and require considerable boiling : so do phloridzine 



* Communicated by the Chemical Society; having been read December 

 2, 1844. 

 t See the preceding volume, p. 546. 

 Phil, Mag. S. 3. Vol. 26. No. 1 72. March 1845. R 



