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POPULAR SCIENCE REVIEW. 
tube (h) ; it escapes at the point (p), which of course is open. When 
the air has been completely expelled the point of the tube is 
sealed by a blowpipe, and the globule of mercury is made to 
occupy the curved end of the tube by simply raising the oppo- 
site extremity. The tube filled with gas is then detached, the 
more capacious portion is immersed in the cistern of mercury, 
fixed in position by a screw, and submitted to pressure. It is 
the water from the hydraulic pump contained in (r) acting on 
the surface of mercury in the cistern, which produces the latter 
effect. Pressures are indicated by the manometer (m'). 
The Liquefaction of Oxygen. — On December 1 6 last M. 
Cailletet gave experimental evidence of the liquefaction of 
oxygen at the Ecole Normale, in Paris. At a temperature of 
— 29° C. ( — 20° F.), and under a pressure of 270 atmospheres, it 
was still in the gaseous state ; but under the influence of a sudden 
expansion, which lowers the temperature about 200° C. (360° F.), 
a sort of mist is produced within the tube, which can be caused 
only by the oxygen being cooled below its critical point, and 
thus assuming the liquid state. 
Nitrogen. — Pure dry nitrogen, compressed to about 200 
atmospheres, at a temperature of + 13° C. (56° F.), if allowed to 
expand rapidly, condenses in a more complete manner ; it first 
presents an appearance like that of spray, consisting of drops of 
an appreciable size, which move from the sides to the centre of 
the tube, forming a vertical column along its axis. The duration 
of this appearance is about three seconds. 
The experiment was repeated on December 30, in the 
presence of many savants , at the laboratory of the Ecole 
Normale. 
Hydrogen. — This substance, which possesses very nearly all 
the properties of a perfect gas, has been considered by most 
philosophers as absolutely incondensable. M. Cailletet, in sub- 
mitting it to the tests which proved so successful with other 
bodies, failed on several occasions to observe any resulting 
phenomena. Several repetitions of his experiments on Dec. 
31, in presence of MM. Ste. Claire Deville, Berth elot, and Mas- 
cart, gave indications of the liquefaction of hydrogen which 
were convincing to those who witnessed them. Pure hydrogen, 
under a pressure of 280 atmospheres, when allowed to expand 
very suddenly, forms an extremely attenuated mist, which in- 
stantly disappears. The production of this mist, notwithstand- 
ing its extreme subtlety, was regarded as an undoubted proof 
that a change to the liquid state had been effected. 
Ai/r . — By the liquefaction of oxygen and nitrogen that of air 
is already demonstrated ; but as it was of interest to actually 
perform the experiment, a portion of air, dried and freed from 
carbonic acid, was submitted to pressure and subsequent ex- 
