176 Dr. J. P. Kuenen on the Condensation and Critical 



5. In order to purify the gas further, it was compressed by 

 means of a Natterer pump into a small copper cylinder fitted 

 with high-pressure stopcocks *. In this process it was once 

 more washed in potash and subsequently dried by calcium 

 chloride and phosphorus pentoxide. Pump and cylinder 

 were previously exhausted by a mercury air-pump. During 

 the compression the copper cylinder was placed in solid 

 carbonic acid. The pressure required to liquefy the gas was 

 pretty high : this shows that it contained a considerable 

 quantity of some permanent gas ; this may have been air 

 Avhich in some unknown manner had got into the gas. By 

 thoroughly boiling the ethane, as much of this admixture was 

 removed as possible. The results obtained with this ethane 

 show that some admixture was left, of about the same amount 

 as in the gas which I used in my former experiments. It is 

 noteworthy that with other substances, such as carbonic acid 

 and nitrous oxide, I always succeeded in removing practically 

 all impurity by this process ; for some reason the same thing 

 seems more difficult with ethane. It is not impossible that 

 ethane forms a mixture of constant boiling-point with the 

 admixture, whatever it may be. 



6. The apparatus which was used for filling the com- 

 pression-tubes with the gases consisted almost of one piece of 

 glass. The mercury air-pump was sealed to the double 

 mercury gasometer which I used for preparing the mixtures 

 (see below). This was connected at the other side with a 

 tube leading to the compression-tube, and another tube 

 through which the purified gas was supplied from the com- 

 pression cylinder. The connexion with the cylinder was 

 made by means of a copper tube, sealed with sealing-wax into 

 a glass tube, the connexion with the compression tube being- 

 made by a ground joint. This was so adjusted, that the tube 

 after being rilled could be turned upright without bsing 

 detached, by which manipulation the tube was shut off at the 

 bottom by some mercury. There are two reasons why I 

 prefer this manner of filling the tubes to sealing them oft. 

 Jn the first place many gases are more or less decom- 

 posed at the temperature of softened glass, and in the second 

 place the sealing place is apt to end in a sharp point, while 

 for the proper action of the little iron stirring-rod which I 

 use inside the tubes, a rounded top is absolutely necessary. 

 It need hardly be added that the apparatus was fitted with a 



* Most of my high-pressure apparatus are manufactured by Gilt ay, 

 Delft, Holland, and are of the same type as those used in the Physical 

 Laboratory at the University of Leyden (Director, Prof. Kimerlingh 

 Onnes"). 



