that connects the Volume of a Liquid with its Temperature. 409 
peratures, where the reduction of temperature had to be artifici- 
ally produced by mixtures of broken ice and muriate of lime, and 
it represents the temperature of the mercury to be higher than 
that of the alcohol. This is precisely what took place in some 
observations I made on the contraction of ether about 20° below 
the atmospheric temperature. A similar deflection, but in a 
greater degree, appears in the ether observations of the same 
authors. They are projected in figs. 5 and 6 to the same scale 
as the others. (See Note G.) 
The application of cold to maintain a constant temperature is 
by no means under the same command as the application of 
heat ; and, besides, conductibility is very much reduced at low 
temperatures. There is an evident dislocation, the law of con- 
tinuity is broken, but it is at the part of the scale where the mode 
of obervation underwent a change. I submit, therefore, that 
the verdict should be against the observations at the lower tem- 
peratures, not against the law of expansion, which, if in fault, 
would cause the trend of the points to have a general curvature 
throughout the range. 
In judging of the evidence afforded by these graphical projec- 
tions, it should be kept in view that the vertical scale magnifies 
the amount of the differences tenfold. The accordance of theor 
with observation is in some casesremarkable. Thus, for 40° C., 
Muncke’s alcohol and Pierre’s ether do not show a difference 
greater than one-sixth of a degree. We have also to keep in 
mind that the power . that reduces the densities to a straight 
trend, is not arbitrarily assumed to suit a particular series of ob- 
servations, but that it is determined @ priori from the vapour, 
If we take any other value of that that which is thus deter- 
mined, the graphical projection of the computed densities shows 
a general bend. If — is too great, the bow is turned downwards ; 
if too small, the bow is turned upwards. The string of the bow 
only makes its appearance when the value of 4 is that deduced 
from the gradient of vapour-density above described. 
§ 16. The time has not perhaps yet arrived for deducing these 
laws of density from the dynamical theory of heat; but if we are 
ever to arrive at a conception of the true ultimate nature of mo- 
lecular force, it seems clear that the inductive path of least difficult 
approach (if not the only one) is that which sets out from the study 
of the gaseous state, and proceeds by way of that of the equili- 
brated condition of saturated vapours in communication with their 
