TIIK COMPOSITION OF WATKK. HYDROGEN 141 



under the influence of chemical attraction, 3 "' which is not only shown 

 from the fact that hydrogen and oxygen (two permanent gases) form 

 liquid water, but also from many phenomena of the absorption of 

 hydrogen. 



Hydrogen is vigorously condensed by certain solids ; for example, 

 by charcoal and by spongy platinum. If apiece of freshly-ignited char- 

 coal be introduced into a cylinder full of hydrogen standing in a 

 mercury bath, then the charcoal absorbs as much as twice its volume 

 of hydrogen Spongy platinum condenses still more hydrogen. But 

 l><illadium, a grey metal which occurs with platinum, absorbs more 

 hydrogen than any other metal. Graham showed that when heated to 

 a red heat and cooled in an atmosphere of hydrogen, palladium retains 

 as much as 600 volumes of hydrogen. When once absorbed it retains 

 the hydrogen at the ordinary temperature, and only parts with it when 

 heated to a red heat. 30 This capacity of certain dense metals for the 

 absorption of hydrogen explains the property of hydrogen of passing 

 through metallic tubes. 37 It is termed occlusion, and presents a 



also the magnitude of the latent heat of evaporation, cohesion, and compressibility under 

 pressure. In this way Van der Waals' formula embraces the gaseous, critical, and liquid 

 states of substances, and shows the connection between them. On this account, although 

 Van der Waals' formula cannot be considered as perfectly general and accurate, yet it is 

 not only very much more exact i\i&npv = RT but is also more comprehensive, because 

 it applies to both gases and liquids. Further research will naturally give further prox- 

 imity to truth, and will show the connection between composition and the constants 

 (a and b) ; but a great scientific progress is seen in this form of the equation of 

 state. 



Clausius (in 1880), taking into consideration the variability of a, in Van der Waals' 

 formula, with the temperature, gave the following equation of state : 



Sarrau applied this formula to Amagat's data for hydrogen, and found a = 0'0551, 

 c = 0-00043, b = G'00089, and therefore calculated its absolute boiling point as 174, and 

 (pc] = 99 atmospheres. But as similar calculations for oxygen ( 105), nitrogen ( 124), 

 and marsh gas ( 76) gave t c higher than it really is, therefore the absolute boiling point 

 of hydrogen must lie below 174. 



55 This and a number of similar cases clearly show how great are the internal 

 chemical forces compared with physical and mechanical forces. 



36 The capacity of palladium to absorb hydrogen, and in so doing to increase in 

 volume, may be easily demonstrated by taking a sheet of palladium varnished on one 

 side, and using it as a cathode. The hydrogen which is evolved by the action of the 

 current is retained by the unvarnished surface, as a consequence of which the sheet curls 

 up. By attaching a pointer (for instance, a quill) to the end of the sheet this bending 

 effect is rendered strikingly evident, and on reversing the current (when oxygen will be 

 evolved and combine with the absorbed hydrogen, forming water) it may be shown that 

 on losing the hydrogen the palladium regains its original fo'rm. 



37 Deville discovered that iron and platinum become pervious to hydrogen at a red 

 heat. He speaks of this in the following terms : ' The permeability of such homogeneous 

 substances as platinum and iron is quite different from the passage of gases through 

 such non-compact substances as clay and graphite. The permeability of metals depends 



