46 
MR, J. J. WATERSTON ON THE PHYSICS OF MEDIA COMPOSED OF 
the ratio of the specific heat of the vapour to that of water. The air follows the line 
RQ (fig. 3) in its expansion, while the vapour is obliged to keep to its line R.T. 
The latent heat of vapours is another subject where there is room for much 
additional research. Dr. Ure, in his £ Dictionary of Chemistry,’ has given a table of 
eight vapours, but none of the liquids appear to have been pure. The ether boiled at 
112°, and the specific gravity of the alcohol was 0 , 825. If a correction is made for 
this want of purity, it is singular that the latent heat of each is almost exactly in the 
inverse ratio of the specific weight of its vapour. This may indicate that the heat 
required to vaporize a molecule of each of these bodies is the same, and amounts to 
3000°, referred to the constant specific heat of a gaseous molecule, which is the proper 
theoretical standard. 
Is this the measure of the force of liquid cohesion ? On the vis viva theory of heat 
this, for 1 lb. of water, amounts to the force required to raise 1 lb. to the height of 
about 680,000 feet. According to Mitscherlich, the vis viva generated by the union 
of the constituent elements of the same quantity of water amounts to ten times this 
force. 
Considerable attention has lately been given to thermo-chemistry ; but it is to 
be regretted that no notice lias been taken of the permanent change of specific 
gravity that is usually found to ensue in chemical mixtures that evolve heat. It 
would be interesting to ascertain if there is any harmonious connection between the 
quantity of heat evolved and the change of atomic volume. Dr. Ure has given a 
curious example of hydrated nitric acid, where the permanent change of volume 
appears to be the same as would be caused by a permanent change of temperature 
equal to the heat evolved. 
Since only two experiments are required to fix a line of vapour on the chart, it 
would not be a very arduous undertaking to accomplish this for all bodies that throw 
off vapours at accessible temperatures. We might then have the means of answering 
the various questions that cannot fail to suggest themselves on looking at the chart; 
and, first of all, do the vapours of arsenic, iodine, camphor, salts of ammonia, and the 
other solids that rise into vapour before becoming liquid, follow the general law ? 
This question has yet to be determined. 
How are the lines of vapour of the simple bodies related to each other ? We have 
only one example as yet, viz., the vapour of mercury by M. Avogadro. It will 
be remarked that the line drawn through MR, the third and the second last obser¬ 
vation, agrees very well with the position of the other points. This line produced 
meets the axis at 50°, which is certainly lower than the temperature assigned by 
Dr. Faraday’s delicate experiment as the point of no vapour. In judging of this 
discrepancy, however, we must recollect that the density is represented not by the 
ordinate to the line of vapour, but to its sixth power; hence, at the temperature 
of 75°, the density indicated by the line on the chart is only 1 2 o.o oUToiroth of the 
