SODIUM 529 



are cooled by the air but remain sufficiently heated to preserve the 

 sodium in a liquid state, and so it does not choke the apparatus, but con- 

 tinually flows from it. The vapours of sodium, condensing in the cooler, 

 flow in the shape of liquid metal into a vessel 

 containing some non- volatile naphtha or hydro- 

 carbon. This is used in order to prevent the 

 sodium oxidising as it issues from the conden- 

 ser at a somewhat high temperature. During 

 the operation it is necessary (with an iron 

 rod) to occasionally clear the pipe which con- 

 ducts the vapours to the condenser, because it HI^HB^^ 



becomes choked up with solid compounds FIG. 72. Donny and Maresca's 



, . sodium condenser, oonsist- 



formed by the sodium. In order to obtain i n g of two cast-iron plates 



, . f !.. v j.i screwed together. 



sodium of a pure quality it is necessary to distil 



it once more, which may even be done in porcelain retorts, but the dis- 

 tillation must be conducted in a stream of some gas on which sodium 

 does not act, for instance in a stream of nitrogen ; carbonic anhydride 

 is not applicable, because sodium partially decomposes it, absorbing the 

 oxygen from it. 



Pure sodium is a lustrous metal, white as silver, soft as wax ; it be- 

 comes brittle in the cold. In ordinary moist air it quickly tarnishes and 

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

 of the water in the air. In completely dry air sodium retains its lustre 

 for an indefinite time. Its density at the ordinary temperature is 

 equal to 0'9S, so that it is lighter than water ; it fuses very easily at a 

 temperature of 95, and distils at a bright red heat (about 900). 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 mer- 

 cury is transformed into vapour. 39 Compounds or solutions of sodium 



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

 solution of th sodium, Berthelot found that for each atom of the sodium in amalgams 

 containing a large amount of mercury (more than 90 p.c.) the amount of heat evolved 

 increases with ihe quantity of sodium, and also that when a composition is reached which 

 approaches NaHg 5 the heat evolved decreases. In the formation of the latter compound, 

 about 18500 calories are evolved; when NaHg 5 is formed, about 14000; and for NaHg 

 about 10000 calories. Kraut and Popp 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 sodium hydroxide until a crystalline 

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

 chamois leather. This amalgam with a solution of potassium hydroxide forms a 

 potassium amalgam, KHg 10 (Crookewitt, Grimaldi). It may be mentioned here that 



VOL. I. M M 



