530 PRINCIPLES OF CHEMISTRY 



in mercury, or amalgams of sodium, even when containing 2 parts of 

 sodium to 100 parts of mercury, are solids. Only those amalgams 

 which are the very poorest in sodium are liquid. Such alloys of sodium 

 with mercury are often used instead of sodium in chemical investiga- 

 tions, because, in combination with mercury sodium is not easily acted 

 on by air, and is a mass heavier than water, and therefore more conve- 

 nient to handle, whilst at the same time it retains the principal pro- 

 perties of sodium. 40 



It is easy to form an alloy of mercury and sodium having a crystal- 

 line structure, and a definite atomic composition NaHg 5 . The alloy of 

 sodium with hydrogen or sodium hydride, Na 2 H, which has the external 

 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 even at higher tem- 

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

 of sodium absorbs as much as 238 vols. 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 this 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. 42 which corresponds with each definite 

 temperature. This confirms the fact, that the formation of substances 

 capable of dissociation is only accomplished within the limits of the 

 dissociation. Sodium hydride melts more easily than sodium itself, and 



the latent heat of fusion (of atomic quantities) of Hg = 360 (Personne), Na = 730 

 (Joannis), and K = 610 calories (Joannis). 



40 Alloys are so similar to solutions (exhibiting such complete parallelism in proper- 

 ties) that they are contained in the same class of so-called indefinite compounds (Chap- 

 ter I.). But in alloys, as substances passing from the liquid to the solid state, it is 

 easier to discover the formation of definite chemical compounds, and therefore, in order 

 to arrive at a correct understanding of solutions, it is very important to study alloys. 

 In addition to this, they are of themselves of considerable interest. The combination of 

 feodium with hydrogen, where the change of physical properties is so evident and where 

 the conservation of chemical properties and easy dissociation are so apparent, ought in 

 this respect to supply much towards an understanding of alloys and also of solutions. 

 Alloys and solutions are homogeneous ; if the decomposition has commenced or increased 

 we may not perceive it, but the products of the splitting up of the alloy Na- 2 H are hetero- 

 geneous ; they are visible. 



41 Potassium forms a similar compound, but lithium, under the same circumstaiu <-s, 

 does not. 



42 Namely, the tension of dissociation of hydrogen, p, in millimetres of mercury, is : 



t = 330 350 380 400 420 430 

 p = 28 57 150 447 752 900 



