Traite Elemen'aire des Reactifs. 331 



29-9 barom. pressure. By our calculation, one gramme of iron 

 should afford only 424 gr. measures of hydrogen. 



Pure zinc must be sought for by reducing to the metallic state the 

 oxide thrown down from the purified sulphate. When this metal is 

 employed to separate copper from its saline solutions, we must re- 

 member, that it precipitates this metal from the nitrate chiefly in 

 the form of a subnitrate. 



In the preparation of lime-water, we are rightly desired to reject 

 the first portions of the solution, which may contain some 

 saline matter. Lime-water is the usual test of corrosive sublimate; 

 but hydriodate of zinc is much better. That water is also the 

 common test of carbonic acid in an alkaline ley. We shall see 

 presently that subacetate of lead is a far more delicate test. The 

 chief employment of magnesia as a re-agent is in vegetable ana- 

 lysis, where it serves conveniently to precipitate the vege- 

 table alkalis from their native combinations in the substance of 

 plants. 



Liquid potash is recommended for detecting the artificial colora- 

 tion of wines. It affords, with these liquids, the following pre- 

 cipitates relative to the different colouring matters employed. 



With the natural principle of wine the prec: 



Berries of yeble 



Indian wood 



Mulberries 



Brazil wood 



Beet 



Turnsole or litmus 

 Myrtle berries 



ipitate is green 

 . violet 

 . violet-red 

 . violet 

 . red 

 . red 



. clear violet 

 . wine-lees co- 

 lour - 



Elder berries .... bluish. 

 This test may possibly be applied with advantage to detect adul- 

 terations in much of the port wine retailed in this kingdom, which 

 is a villanous compoimd of malt spirit and dye-stuffs. 



A separate chapter is allotted to the deuto-chloride and deuto- 

 cyanide of mercury, which bodies might have been better placed 

 among the salts. 



The acids used as re-agents are discussed in the fifth chapter. 

 Strong acetic acid dissolves both gluten and resin. Dilution with 

 water throws down the resin, and saturation with an alkali preci- 

 pitates the gluten. 



Arsenious acid is prescribed as a test of sulphuretted hydrogen ; 

 orpimcnt being formed by the combination. An arsenite is dis- 

 tinguishable from an arseniate by nitric acid, which throws down 

 from a solution of the former arsenious acid in powder. 



They free iron from cobalt by means of oxalic acid ; the oxalate 

 of the latter metal being insoluble. 



Sulphite of lime is enjoined as a substance capable of arresting 

 the fermentation of wines, and other vegetable juices. 



