Properties of Pure Substances : Nitrogen, 9 



The crystals are repeatedly washed with strong hydro- 

 chloric acid and dried hy pressing them into tubes with 

 Kieselguhr. All these operations require to be performed 

 in an atmosphere free from oxygen, and consequently are 

 troublesome. The crystals are of a fine sky-blue colour, and 

 have a really extraordinary affinity for oxygen. For my 

 purpose, however, I disliked the use of the sodium acetate, 

 because I never felt quite sure that I had washed all the 

 resulting acetic acid out of the crystals. I have no reason to 

 believe that, even if acetic acid were present during the 

 absorption of oxygen, any organic compound would be libe- 

 rated, but in the absence of any information I thought it 

 better not to run the risk. I therefore turned my attention to 

 the original blue solution. This of course contains chlorides 

 of zinc and potassium, free hydrochloric acid, hydrogen in 

 solution, and possibly traces of sulphur compounds coming 

 from impurity in the potassium dichromate. It is a question, 

 then, whether during the absorption of oxygen we may not 

 have chlorine or sulphur compounds liberated. Both the acid 

 and zinc were free from arsenic and antimony, so that I had 

 no uneasiness about these substances. I first satisfied myself 

 that chlorine was not given off, by allowing nitrogen with 

 3 per cent, of oxygen to pass through two flasks of chromous 

 chloride, and then through copper sulphate to retain traces of 

 hydrochloric acid, and finally over paper dipped in a solution 

 of iodide of potassium and starch. Though ten litres of gas 

 were passed through, I could detect no change in the starch- 

 paper. Sulphuric acid, when added to the blue liquid, is 

 partly reduced and hydrogen sulphide is given off; conse- 

 quently, since potassium bichromate is generally contaminated 

 with potassium sulphate, which is hard to remove, I had to 

 test whether any sulphur compounds were evolved. For this 

 purpose, and with the arrangement of the last experiment, I 

 passed nitrogen through the liquid and then over filter-paper, 

 saturated with a slightly acid solution of nitrate of silver, but 

 no change of colour could be detected. It remained for me 

 to make sure that in the liquid I intended to use there could 

 be no volatile impurity (the traces of hydrochloric acid itself 

 need not be considered, because hydrochloric acid can be 

 stopped readily by potassium hydrate). 



Before discussing this I must explain my method of pre- 

 paring the blue liquid, which was as follows : — About ten litres 

 of the raw product were prepared in several operations and 

 drawn into one large vessel through a filtering-tube. This 

 tube contained a length of four inches of compressed glass- 

 wool, ana was closed at its lower end by a perforated platinum 





