538 PEINCIPLE8 OF CHEMISTRY 



appearance of a -metal, 41 is a most instructive example of the character* 

 istics of alloys. At the ordinary temperature sodium does not absorb 

 hydrogen, but from 300 to 421 the absorption takes place at the 

 ordinary pressure (and at an increased pressure eveji at higher tem 

 peratures), as shown by Troost and Hautefeuille (1874). One volume 

 of sodium absorbs as much as 238 volumes of hydrogen. The metal 

 increases in volume, and when once formed the alloy can be preserved 

 for some time without change at the ordinary temperature. The 

 appearance of sodium hydride resembles that of sodium itself ; it is 

 as soft as tliis latter, when heated it becomes brittle, and decomposes 

 above 300, evolving hydrogen. In this decomposition all the pheno- 

 mena of dissociation are very clearly shown that is, the hydrogen 

 gas evolved has a definite tension 4a corresponding with- each definite 

 temperature. This confirms the fact that the formation of substances 

 capable of dissociation can only be accomplished within the dissociation 

 limits. Sodium hydride melts more easily than sodium itself, and then 

 does not undergo decomposition if it is in an atmosphere of hydrogen. 

 It oxidises easily in air, but not so easily as potassium hydride. The 

 chemical reactions of sodium are retained in its hydride, and, if we may 

 so express it, they are even increased by the addition of hydrogen. At 

 all events, in the properties of sodium hydride 48 ' we see other properties 

 than in such hydrogen compounds as HC1, H 2 0, H a N, H 4 0, or even 

 in the gaseous metallic hydrides AsH 8 , TeH a . Platinum, palladium, 

 nickel, and iron, in absorbing hydrogen form compounds in which 

 hydrogen is in a similar state. In them, as in sodium' hydride, the 

 hydrogen is compressed, absorbed, pocluded (Chapter II.) * 3bi8 



** Potassium forms a giypilar compound, but lithium, under the same circumstances, 

 does not. 



48 The tension of dissociation of hydrogen p, hi millimetres of mercury, is : 



< = 830 850 400 480* 

 forNaaH p w 28 57 447 910 



forEaB 45 78 548 1100 



48 In general, during the formation of alloys the volumes change very slightly, and 

 therefore from the volume of NaoH some idea may be formed of the volume of 

 hydrogen in a solid or liquid state. Even- Archimedes concluded that there was gold 

 in an alloy of copper and gold* by reason of its volume and density. From the fact that 

 the density of Na-jH is equal to 0*959, it may be seen that the- volume of 47 grams (the 

 gram molecule) of this compound =49'0 c.o. The volume of 46 grams of sodium con- 

 tained in the Na-jH (the density under the same conditions' being 0'9?j is equal to 47*4 

 0.0, Therefore the volume of 1 gram of hydrogen in Na-jH is equal-to 1'6 c.c., and conse- 

 quently the density of metallic hydrogen, or the weight of 1 6.0., approaches 0'6 gram. 

 This density is also proper to the hydrogen alloyed with potassium and palladium. 

 lodging from the scanty information which is at present available, liquid hydrogen near 

 its absolute boiling point (Chapter IL) has a much lower density. 



43 bit -WQ may remark that at low temperatures Na absorbs NH 5 and forms (NH 3 Na) 8 

 (see Chapter VL, Note 14) ; this substance absorbs CO and gives (NaCO)n (Chapter IX., 

 Fote 81), although by itself Na does not combine directly with CO (but K does). 



