220 



REPORT — 1870. 



3. After liquefaction has begun an increase of pressure of 16 atmospheres 

 or more is required to liquefy the whole mass of the nitrous oxide ; for at 

 25°-17 liquefaction began at a pressure of 57'S3 atmospheres, ■whilst the 

 whole was liquid at a pressure of 73*G8 atmospheres. At 32°"2 I found the 

 commencement of liquefaction at a pressure of 67'63 atmospheres, and the 

 terminatiou at a pressure of 84-On atmospheres. For carbonic acid, that was 

 mixed with ^^i^ to y^Vu of air, tlie increase of pressure amounted to 1*5 at- 

 mosphere. Had the gas been pure no increase of pressure could have oc- 

 curred. This shows that a greater quantity of a permanent gas must be 

 mixed with the nitrous oxide ; the variations of the volume of the gas under 

 a given pressure and temperature result perhaps from its whole mass not 

 being homogeneous, as the diminution of the volume is too fast to allow a 

 perfect diffusion of the two gases. 



Tlie gas used for these experiments was prepared from 2)uro nitrate of am- 

 monium. The salt was carefully heated in a tin bath in order to prevent an}^ 

 decomposition of the liberated gas by a too irregular lieating when directly 

 exposed to a flame. It was washed by transmission through a strong solu- 

 tion of caustic potash and dried over sulphuric acid. The caustic potash de- 

 composes any solid particles of the salt that might be carried over mechani- 

 cally and retains the nitric acid, whilst the free ammonia is absorbed by the 

 sulphuric acid. Purified in this manner, the gas was made to pass through 

 the glass tube wherein it was to be compressed. A pressure of about 90 to 

 100 millims. of mercury was required to maintain a moderate current of gas 

 through the capillary bore : this current was continued for five hours or more 

 in order to ensure the complete removal of the air ; the capillary end was then 

 sealed and the other end introduced under mercury. As the experiments 

 with the tube filled in this manner indicated always the presence of a per- 

 manent gas, I tried afterwards to remove the air by exhausting the tube with 

 the air-pump and then to fUl with the gas ; this operation was successively 

 repeated from twenty to thirty times, but with no other result. 



As I could not get the gas pure by heating nitrate of ammonium, I tried 

 to get it from liquid nitrous oxide as it is made in iron bottles in London ; it 

 was probable that the permanent gas would escape first and the nitrons oxide 

 remain pure. This, hoAvever, did not occur, and I got nearly the same result 

 as before. 



In order to prevent diffusion as much as possible, all the caoutchouc joints 

 were besmeared with a solution of tar and asphalt, and the current of gas 

 issued under sulphuric acid. The amount per cent, of this permanent gas 

 was determined in the following manner : — The absoi-ption-tube of Bunsen's 

 absorptiometcr was partly filled under water with nitrous oxide and then left 

 standing three days or longer. The whole of the gas was not absorbed : 



