Properties of Pure Substances : Nitrogen. 13 



seem to be of importance, but it is necessary to get rid of 

 grease. This must be done in such a way as not to waste too 

 much copper. Id order to clean the turnings and gauze they 

 were soldered into a copper tube, and a considerable quantity 

 of shale benzine was distilled and allowed to condense among 

 them. This secures that every part shall be wetted. The 

 process was then continued by gasoline. The gasoline was 

 distilled over the copper to avoid residues, and was finally 

 dried out at about 120° for twelve hours, a good stream of 

 filtered air being drawn over the copper by means of an air- 

 pump. The same process was gone through with alcohol, 

 and finally water. The copper was then washed with dilute 

 chromic acid and digested with strong ammonia for half an 

 hour. All this trouble was gone to, to avoid wasting the 

 turnings, which one has to cut oneself in order to get them 

 in nice long curls, so that they will stand in the bottle and 

 not break up at once and go to the bottom. After about a 

 year's work it was found necessary to renew the copper and 

 ammonia — the former having almost disappeared in a yellow 

 red mud (cuprous oxide ?) which was not further examined — 

 and the ammonia solution had also partially lost its blue 

 colour. Copper strip about 1 millim. thick was now used 

 to replace the gauze and turnings and wire, and was of course 

 strong enough to stand dipping in nitric and sulphuric acid. 

 It is, on the whole, more convenient to use copper in strip 

 than any other form. 



Sulphuric Acid. — This was bought as pure for analysis, 

 and was heated with pure ammonium sulphate for some time 

 before being used. Its purity was, however, of no great 

 moment, except in the case of the final drying-tube P. 



Chromous Chloride. — Already discussed. 



Potassium Hydrate. — During the experiments on the de- 

 composition of ammonium bichromate, I incidentally noticed 

 that all the samples of potash and soda in my possession 

 yielded traces of nitrites, either on simple solution or when 

 distilled with strong sulphuric acid (which was itself free 

 from this impurity) and the distillate reduced by sodium 

 amalgam. The samples of potash which were said to have 

 been purified by solution in alcohol were the worst in this 

 respect, while some rough lumps of commercial caustic potash 

 were nearly free from the impurity in question. I found 

 considerable quantities of nitrites also in samples of potash 

 procured by exposing clean potassium to moist air. I did 

 not try with dry air. After a good deal of trouble I was re- 

 duced to using a solution of a sample of potash in sticks not 

 specially purified, but which only gave the reaction very 



