RESEARCHES UPOIST THE CHEMICAL PROPERTIES OF GASES. 177 



I^one of the preceding reactions of hydrogen appear to fall under the first class. 

 N^early all are of the second class. That reactions such as I have called the third 

 class should occur is difl&calt of explanation, and it is questionable whether a parallel 

 is to be found in the case of ordinary reactions of metallic salts, e. g., precipitation 

 of traces of ferrous sulphide in ferrous chloride solution by hydi-ogen sulphide, or 

 calcium oxalate by ammonium oxalate in dilute acid solution, where a change in 

 the proportion of free acid or alkali may cause the precipitation to become complete. 

 The reduction of ammoniacal silver solution by hydrogen cannot be materially accel- 

 erated by increase of ammonia or other change in the conditions, and thus remains 

 a typical reaction of the third class, no matter how it is carried out. 



2. Reactions at High Temperatures. 



The heat of formation of hydrogen chloride being high (22 calories), it seemed 

 probable that hydrogen should reduce the chlorides of many of the metals at mod- 

 erate temperatures. Small quantities of metallic chlorides were heated in a slow 

 current of hydrogen in a glass tube in the iron oven. The following reactions weie 

 observed : 



TEMPERATURES OF REDUCTION. 



Ruthenium cliloride, anhydrous 190°. 



Gold chloride (obtained by evaporation of a solution of ISTot reduced at 300°. 



gold in aquaregia to dryness). 



Rhodium chloride, anhydrous 200°. 



Platinum chloride (obtained by evaporation of the solu- Gave ofif hydrogen chloride and water at 150^. Not 



lion of platinum in aquaregia to dryness). reduced at 300°. 



Palladium chloride Reduced cold. 



Silver chloride 2700-280°. 



Silver bromide 330O-360o. 



Silver iodide 350O-37CO. 



Mercuric iodide Volatile without reduction. 



In the above experiments the hydrogen, after passing the heated metallic chlo- 

 ride, was conducted into dilute silver solution and the temperature observed at which 

 a precipitation occurred. The reduction of palladium chloride occurs at the ordi- 

 nary temperature. It is an exothermic change, as the following equation shows : 



2 PdClj + 5 H = PdjH * + 4 HCl 

 — (2 X 40 Cal.) + 9.4 Cal. + (4 x 22 Cal.). 



Hence the heat of the completed reaction will be 17.4 calories. 



If hydrogen be passed over palladium chloride contained in a glass tube, an 

 immediate reduction to metallic palladium occurs, attended by evolution of hydro- 



*The composition of palladium hydride is probably PdjH, according to Mendeleef, Principles of Uhemistry. 

 Vol. II, p. 355. 



