124 
PROFESSOR W. RAMSAY AND DR. S. YOUNG 
stance which undoubtedly dissociates into dissimilar molecules, or the dissociation of 
which is undoubted from some change in its physical properties. 
2. Such a study as we propose involves the determination, within the largest 
possible range of temperature, of the expansion of the liquid and of its compressibility; 
of the density of the vapour when unsaturated as well as when in contact with its 
liquid ; and of the heat absorbed in producing change of state from liquid to gas. 
We have already accumulated a number of data dealing with a portion of this 
problem ; and as they are necessarily very voluminous, we deem it advisable to publish 
those bearing on the thermal constants of alcohol. 
3. It is proposed to reserve until later a detailed description of the apparatus with 
which this research was executed. It will be sufficient here to state that it is a 
modification of Andrews’ apparatus, provided with two air-gauges, one for measuring 
low, and the other for measuring high pressures. The temperature of each gauge was 
accurately known, for they were jacketed with running water in which a thermometer 
was immersed. 
These gauges were graduated in millimetres and calibrated by weighing with 
mercury, and were filled with air dried with phosphorus pentoxide. 
In calculating pressure, the following data were required :— 
(1) The temperature and pressure of the air when the gauge was filled. 
(2) The temperature of the air when readings were taken. 
(3) The coefficient of compressibility of dry air, taking into account the deviation 
from Boyle’s law, as determined by Amagat, ‘ Comptes Bendus,’ 99, p. 1153. 
(4) The error of the gauges, corrected by calibration. 
(5) The difference in height of the columns of mercury in the gauges and in the 
tube containing the liquid. 
(6) The vapour-pressure of mercury in the experimental tube. At the highest tem¬ 
perature employed, 246°, it amounts to 68 mms., or about -gwoth of the total pressure 
measured. For this, in many cases, we have applied no correction, for it is doubtful 
whether the mercury can give off vapour when its surface is covered by a long column 
of liquid. From experiments on the vapour-pressure of bromo-naphthalene at 237°, 
at which the vapour-pressure of mercury is 55 mms., it was noticed that a considerable 
time was required for the vapour to rise through a comparatively short column of liquid 
bromo-naphthalene. It is, therefore, probable that the full pressure of mercury vapour 
was not exercised : and the possible error is reduced below the experimental error of 
reading at such high pressures; but in the case of unsaturated vapour-densities, this 
correction was introduced. 
4. The experimental tube, containing the alcohol, confined by mercury, was 
200 mms. long, and was graduated in millimetres, and calibrated by weighing with 
mercury. It was made of lead-glass barometer-tube, with an internal diameter of 
about 2 mms. The coefficent of expansion of this glass was determined between 30° 
and 100°, and was found to be 0'0000257 per degree. In correcting for the meniscus 
of the mercury, the meniscus was regarded as a hemisphere. 
