176 RESEARCHES UPON THE CHEMICAL PROPERTIES OF GASES. 



Permanganate, acidulated with sulphuric acid Bleached slowly. 



Permanganate, alkaline Slowly changes to brown. 



Potassium bichromate, acidulated with sulphuric acid.. Unchanged, cold or at 100^. 



Mercuric chloride Unchanged. 



Osmic acid Unchanged. Prolonged contact iu bright light yields 



traces of reduction after two or three weeks. 

 Ferric chloride Unchanged cold. Traces of reduction to ferrous chloride 



after heating for several hours at 100-^. 



Potassium ferricj^anide Unchanged. 



Ruthenium chloride Unchanged. 



ISTitric acid, fuming* Unchanged. 



Comments. — Russel (t7. Chem. Soc, 2, Yol. XII, p. 3) states that hydrogen 

 reduces silver nitrate solution, nitric acid being at the same time reduced to nitrous 

 acid. Pellet {Compt. Rend., LXXVIII, p. 1132) finds that this reduction is due to 

 the silver salt containing Ag.O in excess, but that perfectly neutral silver nitrate is 

 not altered. In a series of experiments I have obtained results corroborating those 

 of Pellet. Silver nitrate containing a minute trace of free nitric acid is not altered 

 by hydrogen. If some freshly precipitated and washed silver oxide is digested with 

 solution of silver nitrate, and the liquid then filtered, it will have an alkaline reaction 

 towards litmus and is slowly reduced by hydrogen. Boiling the solution with silver 

 oxide increases its alkalinity and also its sensitiveness towards hydrogen. As a 

 reagent for the recognition of hydrogen, it is better that the solution of silver nitrate 

 should be slightly basic (alkaline). 



As regards the action of hydrogen upon ferric chloride, it should be said that 

 mere traces of ferrous chloride are produced, as indicated by a faint change of color 

 upon addition of potassium ferricyanide. 



Free hydrogen has, therefore, a considerable reducing power for some of the 

 more easily reducing metallic salts, which is intensified in some cases by heating 

 to 100°. 



It is convenient to distinguish between three classes of gas reactions as regards 

 intensity. 



Reactions of the first class, in which a change is prompt and quantitative in its 

 results, e. g., when carbon dioxide is brought into contact with soda solution. 



Reactions of the second class, in which a change is slow but no less complete in 

 a somewhat longer interval of time ; e. g., the reduction of platinum chloride solution 

 by hydrogen. 



Reactions of the third class, in which a change is not recognizable until after a 

 considerable interval of time, and the products appearing then are only found in 

 traces, such as the reduction of ferric chloride by hydrogen. 



* Winkler, Zeit. Anal. Chan., 1889, p. 209. 



