SODIUM 63T 



and becomes covered with a film of hydroxide, NaHO, formed at the 

 expense of the water in the air. In perfectly dry air sodium retains 

 its lustre for an indefinite time. Its density at the ordinary tempera- 

 ture is equal to 0-98, so that it is lighter than water ; it fuses very 

 easily at a temperature of 95, and distils at a bright red heat (742* 

 according to Perman, 1889V Scott (1887) determined the density of 

 eodium vapour and found it to be nearly 12 (if H = 1). This shows that 

 its molecule contains one atom (Hke mercury and cadmium) Na. 88 Wi It 

 forms alloys with most metals, combining with them, heat being some- 

 times evolved and sometimes absorbed. Thus, if sodium (having a clean 

 surface) be thrown into mercury, especially when heated, there is a flash, 

 and such a considerable amount of heat is evolved that part of the 

 mercury is transformed into vapour 39 Compounds or solutions of 

 eodium 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 

 investigations, because in combination with mercury sodium is not 

 easily acted on by air, and is heavier than water, and therefore more 

 convenient to handle, whilst at the same time it retains the principal 

 properties of sodium, 40 for instance it decomposes water, forming 

 NaHO. 



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

 line structure, and a definite atomic composition, NaHg 6 . The alloy of 

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



88 MI This is also shown by the fall in the temperature of solidification of tin produced 

 by the addition of sodium (and ajso Al and Zn). Heycock and Neville (1889). 



59 By dissolving sodium amalgams in water and acids, and deducting the heat of 

 solution, of the eodium, Berthelot found that for each atom of the eodium in amalgam* 

 containing a larger amount of mercury than NaHg 5 , the amount of heat evolved increases, 

 after which the heat of .formation falls, and the heat evolved decreases. In the formation 

 of NaHgs about 18,500 calories are evolved ; when NaHg 5 io formed, about 14,000 ; and 

 for NaHg about 10,000 calories. Kraft regarded the definite crystalline amalgam as 

 having the composition of NaHg 6 , but at the present time, in accordance with Grimaldi's 

 results, it is thought to be NaHg 5 . A similar amalgam is very easily obtained if a 

 8 p.c. amalgam be left several days in a solution of eodium hydroxide until a crystalline 

 mass is formed, from which the mercury may be removed by strongly pressing in chamois 

 leather. This amalgam with a eolation of potassium hydroxide forms a potassium 

 amalgam, KHg 10 . It may be mentioned here that the latent heat of fusion (of atomio 

 quantities) of Hg=860 (Personne), Na = 780 (Joannis), and K = 610 calories (Joannis). 



40 Alloys are so similar to solutions (exhibiting sucn complete parallelism in proper* 

 ties) -that they are included in the same class of so-called indefinite compounds. But'Ta 

 alloys, as substances passing from the liquid to the solid state, it is easier to discover tho 

 formation of definite chemical compounds. Besides the alloys of Na with Hg, those with 

 ttn (Bailey 1892 found NaaSn), lead (NaPb), bismuth (NaaBi), &c. (Joannia 1892 and otheri) 

 have been investigated. 



