CONDUCTIVITY OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. '23 



air and not allowed to come into contact with glass, it was free, at least initially, 

 from carbonate and silicate. In one experiment the sodium left behind in the cradle 

 .1 few milligrammes of a black granular metallic residue (possibly iron sawdust 

 introduced in opening the case in which the sodium was packed) ; although the 

 quantity was probably less than O'Ol per cent, of the weight of sodium, this solution 

 was regarded as of unknown strength and was not used as a standard ; in all the 

 remaining experiments the only residue was occasionally a brown stain on the silver 

 cradle. The sodium was therefore free from all but the merest trace of heavy 

 metals. 



A further test was made for the presence of alkaline earths. About half a litre of 

 concentrated solution had been allowed to become slightly carbonated, and was 

 decanted from the carbonate tliat had been precipitated at the bottom of the bottle ; 

 on adding water the precipitate was found to be completely soluble, and there was 

 therefore no appreciable quantity of calcium or magnesium in the metallic sodium or 

 in the hydroxide prepared from it. The presence of a small quantity of potassium or 

 lithium would have had little influence on the subsequent measurements, but a 

 spectroscopic test did not reveal the presence of these elements, and we were therefore 

 led to conclude that the metal we had used as our standard was free not only from 

 acidic impurities, but also from all but traces of foreign metals. 



More difficulty was experienced in ascertaining whether the metal might not 

 be alloyed with carbon or hydrogen in the form of carbide or hydride. When 

 decomposed by water, these substances would give rise to volatile products and would 

 not affect the purity of the solutions, but if present in considerable quantities might 

 lead to serious errors in the concentrations. Sodium carbide does not appear to be 

 soluble in metallic sodium, but under certain conditions large quantities of hydrogen 

 may be absorbed. The concordance of our measurements for the density oi 

 50-per cent, solutions of sodium hydroxide prepared from different samples of sodium 

 seemed to indicate that there could not be any large quantity of volatile impurity 

 in the metal, but on the advice of Dr. SCOTT we tried an additional method ot 

 purification by heating a sample of sodium in an iron ladle in such a way as to 

 expel any carbide or hydride that might initially be present. The metal was heated 

 until it caught fire and was then poured into melted paraffin wax, but when 

 suKsequi'iitly converted in the usual way into hydroxide it gave a normal value for 

 the density, and did not appear to have been altered in any way by the roasting. 

 As a matter of fact, metallic sodium is now made almost exclusively by electrolytic 

 processes and no longer contains the black explosive carbide that was formerly 

 associated with it ; even when crude sodium is heated, the black residue that is left 

 in the ladle appears to consist merely of a little lamp-black from the burning 

 petroleum, and if clean metal is used no black scum is produced. A test for acetylene 

 in the gases produced by the action of steam on the metal gave a negative result. 



An attempt was made to detect the presence of hydride in the metal by 



