86 HYDROMETALLURGY. [ill. 



tive. Few improvements have been recently made in this art, 

 but we hope to see most colors copied by photography, and to 

 witness its more direct application to engraving by combination 

 ■\Tith the galvanotype. 



According to Schonbein (Pogg. An. Ixxiii.), starch-paste, 

 mixed with freshly-made iodide of lead, so as to give it an 

 intense yellow color, is the most susceptible of all substances 

 to the influence of light. 



Jlyposulpliite of Soda. — Faget's process for making the 

 hyposulphite of soda (Journ. de Pharm. 1849) yields a more 

 uniform and purer article than that obtained by the usual 

 methods. It consists in boiling the neutral sulphite with sul- 

 phur, by which means nearly all the soda is converted into 

 hyposulphite. The author prepares the neutral sulphite by 

 mixing a solution of carbonate of soda with an equal volume 

 of the same solution previously saturated with sulphurous acid 

 gas. The resulting compound is an alkaline bisulphite, with 

 an excess of sulphurous acid held in solution by the water of 

 the liquid. After the entire expulsion of this excess of sul- 

 phurous acid by boiling, sulphur is added and the heat con- 

 tinued. 



Plessy purifies this salt by melting it in its water of 

 crystallization, evaporating slightly, and setting aside to cool. 

 The hyposulphite crystallizes, and the impurities remain in 

 the mother water. 



For the preparation of hyposulphite of soda, see Lond. 

 Journ. 1849, p. 129. 



Iodine. — Niepce de St. Victor has discovered two properties 

 of the vapor of iodine, which promise an extension of pho- 

 tography. The first property is, that it will deposit upon the 

 lines of an engraving, whether executed with printer's ink, 

 Indian ink, ink without gum, or red-lead. Before iodizing, it 

 is better to pass the paper through ammonia, then through 

 water acidulated with sulphuric, chlorohydric, or nitric acid, 

 and dry it. The second property is, its depositing itself upon 

 the projecting parts of embossed plates. (See Chevreul's 

 Report on this subject, in Comptes Rendus, xxv. 785.) 



