178 Dr. Kuenen on the Critical Phenomena of 



with an air-pump, a sort of mercury -valve being interposed 

 in order to prevent air from returning to the liquid. 



Preparation of the Mixtures, 



10. The substances thus purified could be mixed in any 

 proportion by means of my mixing apparatus, for the descrip- 

 tion of which I refer to previous investigations*. At present 

 the whole apparatus consists of glass and copper only : the 

 exhaustion was effected by a mercury air-pump. 



Method of the Observations. 



11. For the observations I made use of Ducretet's appa- 

 ratus!. The glass tubes in which the gases were to be 

 compressed were connected with the above apparatus by 

 means of ground joints, and repeatedly exhausted and filled 

 with the gases. 



12. The temperatures, all of them ranging between 0° and 

 40° C, were obtained b} r means of a continuous current ot 

 water through a tube surrounding the experimental tubes. 



13. My stirring apparatus J, which was continually used 

 during the observations, had been slightly modified. The 

 little piece of iron with its knobs of enamel within the high- 

 pressure tubes was the same as before; but the electromagnet, 

 which used to slide over the tube and accordingly within the 

 water-vessel, had been replaced by a larger bobbin sliding 

 over the outside of the water-vessel. In this manner the 

 influence of the temperature of the bobbin upon the tempera- 

 ture of the water, the constancy of which used to be disturbed 

 during the acting of the apparatus, was removed. The bobbin 

 is worked by four Bunsen cells or two accumulators. 



Results obtained with Ethane. 



14. In order to test the purity of the substances they were 

 at first investigated separately. The gases appeared still to 

 contain a little impurity : there was a slight increase of pres- 

 sure during the process of condensation, for the ethane at 20°, 

 0*56 atmosphere. The amount of impurity of a gas corre- 

 sponding to a definite increase of pressure at a definite 

 temperature is difficult to decide, and entirely depends upon 

 the substance itself and the admixture. In preparing the gas 

 a second time, the liquid ethane was cooled in liquid ethylene 

 boiling at a low pressure ( — 110° C), itself being con- 

 nected with an air-pump as in the case of nitrous oxide. The 

 ethane obtained in this manner was found to be a little purer 



* Kuenen, /. c. p. 369. t L- ft pp. 357, 366. \ L. c. pp. 372 sqq. 



