156 
POPULAR SCIENCE REVIEW. 
600 atmospheres, and kept at a temperature of — 100° C. or 
— 140°C. (— 148° F. — 220° F.),and it is allowed to expand to 
the atmospheric pressure, one of two things takes place : either 
the gas, obeying the force of cohesion, liquefies, and yields its 
heat of condensation to the portion of the gas which expands and 
loses itself in the gaseous form ; or, on the hypothesis that cohe- 
sion is not a general law, the gas must pass to the absolute zero 
and become inert ; that is to say, an impalpable powder. The 
work done by expansion will not be possible, and the loss 
of heat will be absolute.” 
Struck with the truth of this alternative, which is rendered 
certain by thermo-dynamic equations, based upon accurate data, 
M. Pictet devised a mechanical arrangement which entirely 
satisfies these various conditions, and which has been the means 
of experimentally demonstrating the fact that molecular cohesion 
is a general property of bodies, to which there is no exception. 
While M. Pictet, of Geneva, has been engaged for more than 
three years with the solution of this problem, M. Cailletet, of 
Chatillon-sur-Seine, has devised an apparatus for the same pur- 
pose. This very beautiful instrument of M. Cailletet has been 
the first to compel the liquefaction of a so-called permanent gas, 
viz., nitric oxide. There soon followed a simultaneous lique- 
faction of oxygen by both experimenters. While, however, M. 
Cailletet obtains a visible indication of liquefaction with his 
instrument, M. Pictet produces a stream of liquid which may 
be submitted to examination. 
M. Cailletefs Apparatus . — There are two forms of this in- 
strument both the same in principle, the large one in use at 
Chatillon-sur-Seine ; and a smaller and very portable apparatus, 
constructed by M. Ducretet, of Paris, of which fig. 1 is a repre- 
sentation. 
The gas to be compressed is contained in the glass tube (t t), 
the lower end of which is plunged into mercury, contained in 
the cistern (b), which is constructed by boring out a block of 
wrought iron so as to leave it as a cylinder with very thick 
walls. The lower point of the glass tube is curved, and in the 
bend a little mercury is placed, to keep out atmospheric air and 
prevent the escape of any gas before the tube is fixed in its 
place. The upper part of (t) is visible, and is surrounded by a 
jacket (m), containing a refrigerating liquid, such as liquid sul- 
phurous acid, liquid nitrous oxide, or even iced water. Around 
the jacket (m) is placed a bell-jar (c), which contains some desic- 
cating material, such as oil of vitriol or chloride of calcium, to 
dry the air, and so prevent the formation of a dew or deposition 
of hoar-frost on the glass. Should the tube break, these sur- 
rounding vessels would prevent any dangerous result. The com- 
pression in this instrument is obtained by hydraulic power, while 
