478 PROFESSOR W. RAMSAY AND DR. S. YOUNG ON VAPOUR-PRESSURE. 
marked manner; and that, indeed, the curves representing vapour-pressure of solid 
and liquid below the melting-point are quite distinct: and (3) that, with water, as 
Professor Thomson predicted, this difference is calculable from the known heats of 
vaporisation and fusion. It is to be regretted that similar data as regards acetic acid 
are not sufficiently complete or reliable to enable a similar calculation to be made. 
Addendum. 
A neiv method of determining vapour-pressures of solids and liquids. 
As this method has been fully described in this and in a previous paper (Phil. Trans., 
1884, Part I., p. 37) it will not be necessary to enter into a detailed description of it. 
Our experience of the method has shown us that the results which it gives are more 
trustworthy, as well as more easily obtained than those which the older method 
yields ; hence it may be well to point out its advantages. 
1. There is no necessity to heat the whole apparatus to a known and uniform 
temperature. It is only necessary that the temperature of the bath should be kept 
30° to 40° higher than that registered by the thermometer. 
2. The temperature of the gauge and barometer are the same, and involve only a 
simple and small correction for the temperature of the column of mercury. The 
vapour-pressure of mercury is not introduced. 
3. The gauge and barometer stand in the same trough, which may be placed at a 
distance from the still, and hence they are not subject to change of temperature. 
4. The readings are more accurate, because the gauge and barometer need not be 
jacketed, and the scale may be placed immediately behind them. 
5. The very great difficulty of filling an experimental tube, so as to ensure absence 
of air and moisture, is avoided. 
6. In our apparatus, the temperature is dependent on the pressure, instead of the 
pressure being dependent on the temperature, and the pressure may easily be reduced 
or increased at will, and is of necessity constant throughout the whole apparatus. 
7. A very much larger number of observations may be taken in the same time, and 
the influence of the errors of experiment, themselves much smaller, is thereby 
greatly reduced. 
8. The labour involved in applying the corrections necessary when a barometer 
tube is used is avoided. 
