128 Mr. J. D. Hamilton Dickson on the 
•2429 to *2881, was *2593. There was some discrepancy in a 
similar calculation for carbonic acid, but not enough to 
reject the claim that his theory was substantiated by' its 
coincidence with experiment. Shortly after van der Waals's 
thesis was published Clausius proposed a modification of it 
in two directions, the principal one being that the molecular 
attraction constant should vary inversely as the temperature. 
On this assumption, for small pressures, we get the relation 
jc 4=£- 6; (o) 
thus introducing the inverse square of the temperature, 
in accordance with Joule and Kelvin's experiments and 
Rankine's theoretical equation. 
It will not be a matter of surprise that two formulas 
so different should both approximately satisfy the experi- 
mental results, when we consider that on the p, t plane 
these results would be plotted within a range of some 
4 or 5 atmospheres and some 80 degrees of temperature. 
Two such curves, threading their way for the short distance 
necessary among the experimental points plotted on this 
area, would practically coincide. 
Great developments of the experimental methods of Cail- 
letet and Pictet have been made in the last 25 years, notably 
by Dewar, Olszewski, and Linde. To the first of these, 
by the invention of the vacuum-jacketed flask and by his 
subsequent masterly use of it, we owe the production of 
liquid oxygen and hydrogen in bulk under atmospheric 
pressure, and also solid oxygen, air, and hydrogen, thus 
reaching the low.est temperature yet attained ; while, on the 
other hand, the last-named physicist has devised means to 
produce liquid air and oxygen commercially. Olszewski, 
in particular, has studied the lowering of temperature by 
expansion and has, for his researches, modified in some 
important points both the Joule and the Joule -Kelvin 
experiments. In the Olszewski experiment a finite time is 
occupied in the release of the gas under pressure, thus 
differing from the method followed by Joule. There is 
also a departure from the Joule-Thomson porous-plug expe- 
riment in employing a great difference of pressures, and 
(at least as far as can be made out from published papers) 
in restoring the small aperture in place of the porous plug. 
The questions arise : Is the theory of the Olszewski expe- 
riment the same as that of the Joule-Kelvin experiment ? 
If not, what is the theory ? And with these questions there 
