SCIENTIFIC SUMMARY. 
79 
driven slowly back by mercury, cooled to 0° in a capillary tube maintained at 
— 23°. Tbe mercury, which transmits the pressure of the hydraulic press, does 
not impoverish the gaseous mixture as quickly as one might fear. A solid 
glaze is formed on the surface of the metal which rapidly limits the action ; 
the heating of the gas during the compression is more formidable. In spite 
of these difficulties one succeeds in increasing the tension of the ozone to a 
considerable degree. From the first working of the piston, the capillary tube 
becomes of an azure blue ; this coloration is intensified in' proportion as one 
reduces the volume of gas ; and if the tension of the ozone is brought by the 
compression to be one of several atmospheres, the gas is indigo blue, and the 
meniscus of mercury, seen through the gas, is then of a steely blue. The 
blue colour of the gas becomes less intense, and the mercury regains its usual 
metallic appearance, as soon as one diminishes the tension of the ozone. 
The preceding mixture contains enough ozone for one to observe a thick, 
white mist at the moment of the expansion which succeeds a compression of 
75 atmospheres. It is not then necessary to compress the ozonized oxygen as 
much as pure oxygen (300 atmospheres) in order to determine by a sudden 
expansion the momentary formation of a mist, a certain sign of liquefaction 
or even of solidification. 
A comparative study of the mixtures of oxygen and ozone, and those of 
oxygen and carbonic acid, shows that in very comparable conditions the ex- 
pansion ought to be sensibly stronger with the ozone than with the carbonic 
acid, for one to begin to perceive a mist. Ozone would then be a little easier 
to liquefy than carbonic acid. 
The mixture of oxygen and ozone contains an explosive gas, and should 
always be compressed slowly and cooled ; for if one does not not meet these 
conditions, the ozone decomposes with an escape of heat and light, and there 
is a loud report accompanied by a flash of yellowish light. 
Berthelot has found that the transformation of oxygen into ozone absorbs 
14 cal., equivalent to 8 (O 3 = 24 gr.) ; ozone may therefore be placed beside ex- 
plosive gases ; the experiments show that, like them, this body is susceptible 
of sudden decomposition. 
One may also observe a portion of these new facts by compressing 
oxygen which has gone slowly through the electric apparatus at ordinary 
temperature ; for if we compress this gas rapidly in a capillary tube, placed 
in some water at 25°, we often destroy the ozone with explosion ; but if this 
same gas is cooled at — 23°, the ozone which it contains may be brought to 
a tension of 10 atmospheres, and may be preserved for hours in these con- 
ditions of temperature and pressure, if the gas is separated from the mercury 
by a column of sulphuric acid. 
It is ascertained, then, almost equally clearly as in the preceding experi- 
ment, that ozone is a gas of a beautiful azure blue, for its colour is so 
intense when one increases its density tenfold, that it could be seen in a 
tube of 0 m, 001 internal diameter, when operating in a very poorly lighted 
room in the laboratory of l’Ecole Normale. 
It is therefore established that ozone under a strong tension is a coloured 
gas : but may the same be said of ozone at the tension of some millimetres ? 
The blue colour characterizes ozone as surely as its smell; for at all the 
tensions we find it on examining the gas to a sufficient depth. It suffices in 
