I. A number of years ago, it was shown by Ramsay and Young,* that for any pair of closely related substances, the ratio of the absolute temperatures corresponding to equal vapour pressures is constant, *>., Ti/T'i = T._/T'._>. In the case of substances not closely related, it was shown, generally, that the relation between the absolute temperatures at which the two substances have equal vapour pressures could, with very great approximation, be expressed by the equation, R = R' + c (f - i}, R being the ratio of the absolute temperatures, T T' or T'/T, at which the substances both have any given vapour pressure ; K is a constant which may be or a small positive or negative numl>er; and /' and / are the temperatures of one of the substances corresponding to these two vapour pressures. Or, since the variation of the ratio R from constancy was found to be practically a linear function of the temperature, we may also express the above relationship by means of the equation T/T' = a + bT. In this equation n is of the order unity, and l> of the order O'OOl to O0001, and -b is equal to -he of the pre- vious equation. It was further shown by Ramsay and Young that by means of the above equation, if the vapour pressure curve of one sub- stance is known, the vapour pressure curve of any other substance can be calculated from the values, at any two temperatures, of the vapour pressure of that substance. The above relationship was tested by Ramsay and Young in the case of 23 pairs of substances, and has also been found to hold up to the critical pressure.!
The degree of success with which the method can be used, is shown by Table I, which I take from the memoir of Ramsay and Young above mentioned.
At the suggestion of Professor Ramsay, I undertook the investiga- tion of whether any similar method could be obtained for the calcula- tion of solubilities ; and I have found that a precisely similar equation to that of Ramsay and Young connects the absolute temperatures at which two substances have equal solubilities ; that, therefore, the equa- tion R = R' + c (f - t) holds good with respect to the solubilities of any two substances. In this equation R and R' now denote the ratios of the absolute temperatures at which the substances have equal solubili-
- ' Phil. Mag.,' 1886, vol. 21, p. 33.
t In this way the vapour pressures of argon, krypton, and xenon have been calculated by Ramsay and Travers ('Ph.il. Trans.,' A, vol. 197, 1901, p. 47).
