4 Prof. K. Threlfall on the Electrical 



case, and particularly in places where they can become hot. 

 One never feels certain, moreover, that a red-hot tube, even 

 of glazed porcelain, is absolutely impermeable to furnace- 

 gases. If one uses a bath of magnesia this difficulty may 

 perhaps be overcome, but there remains the difficulty of 

 making proper connexions. 



If hard glass be employed instead of porcelain, then there 

 is insecurity in the joints between it and the glass used for 

 the remainder of the apparatus. Metal tubes present similar 

 difficulties with regard to the junctions, and are besides more 

 or less porous at high temperatures. Another grave dis- 

 advantage is that, whether porcelain or glass tubes be used, 

 there is always a chance of a crack occurring, and perhaps 

 escaping notice. 



The reduction of the copper also presents great difficulty: 

 it is more difficult to get a strong stream of really pure 

 hydrogen or carbon monoxide than to get the nitrogen ; 

 and any sulphur absorbed by the copper is a permanent 

 disadvantage ; for, as metallurgists know, copper contain- 

 ing sulphur may be oxidized and reduced many times and 

 yet at each oxidation some sulphur will burn out. The 

 reason is, of course, that sulphide of copper heated in air 

 forms some sulphate of copper as well as sulphur dioxide. 

 On the next reduction sulphide of copper is re-formed, and 

 then, on passing air over the mixture, the process of incom- 

 plete oxidation is repeated. Judging by a discussion which 

 took place in Section A of the British Association not long- 

 ago, these simple facts are not as widely known as might 

 have been expected. I know of no reagent which will absorb 

 sulphur dioxide so as to form an absolutely stable compound, 

 and which is itself easy to prepare in a state of sufficient 

 purity to be aboA~e suspicion of giving off foreign matters to 

 the nitrogen. Of course it may be argued that a large 

 number of vessels containing, say, a solution of caustic potash 

 might be used, the second retaining the sulphur dioxide given 

 off from the first, and so on ; but this device is obviously 

 unsuited for a continuous process, where the reagents must 

 be untouched for weeks or months. 



Finally, I gave the process a careful trial, and found it 

 unsatisfactory from the causes mentioned ; and, as a matter 

 of fact, the trouble of preparing hydrogen or carbon monoxide 

 in a sufficient state of purity to reduce the copper without 

 contaminating it with sulphur or chlorine is at least as great 

 as the trouble of preparing nitrogen itself. 



With regard to the method of passing air over melted 

 white phosphorus, the manipulation of phosphorus is always 



