436 Proceedings of Royal Society of Edinburgh. [sess. 
different samples of tlie gas were weighed. The volume of the 
glass bulbs used for this purpose was about 130 c.c. Mixture a 
could not he weighed, as no sufficient quantity had been kept for 
the purpose. Mixture b and c, as well as a fourth sample (d), 
were weighed and compared with air. They gave the following 
results : — 
Air, 28-88 
Mixture c ( = a), 30‘73 . . cr. temp, . . . 35'75 
Mixture b, 31 '37 „ ... 39 *25 
Mixture d, 32 '52 
The normal weight for ethane would have been 30. If we 
extrapolate the critical temperature of pure ethane from the values 
for c (practically the same as a) and b, we find 3T8° C., which is 
in good agreement with the value obtained for almost pure ethane 
prepared from sodium acetate. 
The substance mixed with ethane, as prepared by this method, 
is evidently heavier than ethane. Moreover, it lowers the con- 
densation pressures and raises the critical temperature (fig. 3). 
All this is compatible with the supposition that the admixture is 
butane — a very probable supposition, when we consider the 
possible chemical reactions taking place in the preparation. The 
critical constants of butane are unknown, but the critical tem- 
perature must be somewhere about 150° C., the point for propane 
being 100° C. (Olszewski, Haenlen), and for pentane about 197° C. 
(S. Young). The same substance seems to have occasioned 
Haenlen’s abnormal results (a higher critical temperature and 
lower pressures), and possibly the same is true for Dewar’s ethane. 
Supposing that the impurity is butane, the weights of the mix- 
tures show that a and b contained 2 ’5 and 5 per cent, butane 
respectively, or 4‘7 and 9*2 per cent, when expressed in parts by 
weight, quantities the presence of which could not be very easily 
detected by gas-analysis. 
It is not impossible that by discarding the scrubber where the 
metal is apt to act upon the iodide if too little alcohol is 
present, a purer product may be obtained, but now that the forma- 
tion of butane is once proved, and as it can hardly be entirely 
prevented, the method loses much of its importance for the prepara- 
tion of a pure gas. The separation of two hydrocarbons like 
