In a paper on mixtures of ethane and nitrous oxide, read before the Physical Society of London in May 1895, I communicated a set of observations with regard to the condensation and the critical state of ethane. This substance was prepared by electrolysing sodium acetate: it was purified with fuming sulphuric acid, caustic soda, and phosphorus pentoxide, and condensed in a small copper cylinder, where it was subsequently boiled at low temperature in order to expel all permanent gas. The ethane with which the glass compression-tubes were filled was drawn from the liquid contained in the copper cylinder. Its condensation-pressures and critical constants are contained in the following table, which is taken from the paper mentioned (comp. fig. 3):—
The values of p and of the critical constants in Table I. are not absolutely correct. The observations showed the substance to contain some impurity, the condensation-pressures not being quite constant, but showing a slight increase, as is always the case for mixtures. The difference of the pressure at the beginning and at the end of the condensation amounted to 0·43 atmospheres at 15° C. It is, however, unlikely that this impurity in the ethane can have affected the values for p by more than a few tenths of an atmosphere. Nor will the value for the critical temperature differ by more than 0·1 or 0·27° C. from the true value, considering that Andrews' carbonic acid showed changes in vapour-pressure of over two atmospheres under the same circumstances, and that his value for the critical temperature, 30·9° C., is only about 0·4° C. too low.