880 PRINCIPLES OP CHEMISTRY 



rhodium and indium in dissolving in aqua regia also form RhCl 4 and 



is precipitated by potassium iodide or mercuric cyanide. Wilm (1881) showed, that 

 palladium may be separated from an impure solution by saturating it with ammonia; all 

 the iron present is thus precipitated, and, after filtering, the addition of hydrochloric 

 acid to the filtrate gives a yellow precipitate of an ammonio-palladium compound^ 

 PdCIa,2NH 3 , whilst nearly all the other metals remain in solution. Metallic palladium 

 is obtained by igniting the aromonio-compound or the cyanide, PdC 2 N 3 . It occurs 

 native- although rarely, and is a metal of a whiter colour than platinum, sp. gr. 11'4, 

 melts at about 1,500 ; it is much more volatile than platinum, partially oxidises on the 

 surface when heated (Wilm obtained spongy palladium by igniting PdCl a ,2NH 3 , and 

 observed that it gives PdO when ignited in oxygen, and that on further ignition thia 

 oxide forms a mixture of Pd a O and Pd), and loses its absorbed oxygen on a further rise 

 of temperature. It does not blacken or tarnish (does not absorb sulphur) in the air at 

 the ordinary temperature, and is therefore better suited than silver for astronomical and 

 other instruments In which fln.e divisions have to be engraved on a white metaj, in order 

 that the fine lines should be clearly visible. The most remarkable property of palladium, 

 discovered by Graham, consists in its capacity for absorbing a large amount of hydrogen. 

 Ignited palladium absorbs as much as 940 volumes of hydrogen, or about 0'7 p.c. of its 

 own weight, which closely approaches to the formation of the compound Pd 5 H 3r and- 

 probably indicates the formation of palladium hydride, Pd 2 H. This absorption 

 also takes place at the ordinary temperaturefor example, when palladium serves as 

 an electrode at which hydrogen is evolved. In absorbing the hydrogen, the palladium 

 does not change in appearance, and retains all its metallic properties, only its volume 

 increases by about 10 p.c. that is, the hydrogen pushes out and separates the atoms of 

 the palladium from each other, and is itself compressed to ^ of its volume. This com- 

 pression indicates a great force of chemical attraction, and is accompanied by the evolu- 

 tion of heat (Chapter II., Note 88). The absorption of 1 grm. of hydrogen by metallic 

 palladium (Favre) is accompanied by the evolution of 4'2 thousand calories (for Pt 20, 

 for Na 13, for K 10 thousand junits of 'heat), trooat showed that the dissociation 

 pressure of palladium hydride is inconsiderable at the ordinary temperature, but reaches 

 the atmospheric pressure at about 140. This subject was subsequently investigated by 

 A. A. Cracow of 8*. Petersburg -(1894), who showed that at first the absorption of 

 hydrogen by the palladium proceeds like solution, according to the law of Dalton and 

 Henry, but that towards the end.it proceeds hire a dissociation phenomenon in definite 

 compounds; this forms another link between the phenomenon of solution and of the 

 formation of definite atomic compounds. Cracow's observations for a temperature 18, 

 showed that the electro-conductivity and tension vary until a compound PdjH is reached, 

 and namely, that the tension p rises with the volume v of hydrogen absorbed, according 

 fb the law of Dalton and Henry for instance) for 



p = 2~l 8'2 5-5 7-7 mm, 



v=14 20 84 47 



The maximum tension at 18 is 9 mm. At a temperature of about 140 (in the vapour of 

 xylene) the maximum tension is about 760mm., and when v = 10-50 vols. the tension 

 (according to Cracow's experiments) stands at 90 450 mm. that is, increases in pro- 

 portion to the volume of hydrogen absorbed. But from the point of view of chemical 

 mechanics it is especially important to remark that Moutier clearly showed, through 

 palladium hydride, the similarity of the phenomena which proceed in evaporation and 

 dissociation, which fact Henri Sainte-Claire Deville placed as a fundamental proposition 

 in the theory of dissociation. It is possible upon the basis of the second law of the 

 theory of heat, according to the law of the variation of the tension p of evaporation with 

 the temperature T (counted from 273), to calculate the latent heat of evaporation 

 L {see works on physics) because 424 L=*T (1/d-l/D) dp/dt, where d and D are the 

 weights of cubic measures of the gas (vapour) and liquid. (Thus, for instance, for 

 r, when <c B lOO,T=873, d = 0'603,D= 960, dp/dt~ 0'027 m., 18,596=867, L530, 



