6 Prof. R. Threlfall on the Electrical 



resulting bichromate of ammonium coming from the small 

 quantity of ammonia which was not destroyed by the oxygen 

 did not differ in behaviour from the other samples. It may 

 be noted here that Stas found (loc. cit.) that purified ammonia 

 has a much less unpleasant smell than ordinary ammonia : 

 careful trial, however, failed to enable me to detect any differ- 

 ence between the smell of my purest ammonia and that 

 obtained by boiling so-called pure solution of ammonia obtained 

 from the dealers. I conclude that the "pure ammonia" of 

 commerce is much purer now-a-days than it was thirty years 

 ago. 



The chromic acid was made from repeatedly crystalli'zed 

 bichromate of potash, bought as pure, and the sulphuric acid 

 which is sold for analytical purposes. The resulting chromic 

 acid was washed free from sulphuric acid by strong nitric acid, 

 distilled for the purpose. I found incidentally that the addition 

 of a little chromic acid to the mixture of nitric and sulphuric 

 acid in the retort appeared to have a good effect in diminish- 

 ing the quantity of nitrous acid which generally accompanies 

 "fuming" nitric acid. The washed chromic acid was dried on 

 tiles in the usual way, and heated with great care and continual 

 stirring until it partly melted ; but even so it was slightly 

 decomposed. Of course the greater portion of the nitric acid 

 had been previously got rid of by heating the chromic acid on 

 a water-bath in a partly exhausted retort with ground-on 

 condenser, the latter containing some sticks of potash. The 

 chromic acid being partially decomposed gave a solution, 

 which had to be filtered through glass-wool before it was 

 used to absorb the ammonia. This solution was tested by 

 distilling, the distillate being tested for nitrites both before 

 and after reduction with sodium amalgam. The result of 

 these tests was to show that either the solution contained no 

 nitro-compounds, or, if so, not more than are contained in the 

 same quantity of platinum distilled water. The bichromate 

 of ammonia made from this was recrystallized three times 

 from platinum distilled water. Some small samples were 

 crystallized up to six times, but did not differ from the results 

 of the third crystallization. All these samples, when heated 

 either in air or in vacuo, gave off nitrous compounds, and 

 did not behave in any way differently to the sample which 

 had been bought as pure. I therefore conclude that the 

 normal decomposition of ammonium bichromate is complex, 

 and that it is not a good substance to use as a source of 

 nitrogen. 



I will now describe a method of procuring nitrogen which 

 has not been mentioned hitherto, which lends itself remark- 

 ably well to a continuous process, and which is almost, but not 



