80 
POPULAR SCIENCE REVIEW. 
order to make this manifest, to interpose between the eye and a white sur- 
face a tube of 1 m. in length, traversed by the current which has passed 
through the electric apparatus of Berthelot. The colour which the gas then 
possesses reminds one of the blue colour of the sky ; this blue is more or less 
deep in proportion to the time the oxygen has remained in the apparatus, 
and is consequently more or less rich in ozone. As soon as one stops the 
electric tube, the coloration disappears, the ozonized oxygen being replaced by 
pure oxygen. 
Ice at High Temperature . — A short communication on this subject to the 
Chemical News by T. Carnelley (vol. xlii. 130) draws attention to the follow- 
ing points. Numerous experiments which have been made during the last 
weeks on the boiling-point of bodies under reduced pressure have led to the 
following conclusions respecting the conditions which are necessary for a 
body to exist in the fluid condition. They are as follows : (1) In order to 
convert a gas into a liquid, the temperature must be below a certain point 
— a point which Andrews termed the critical temperature of the substance ; 
otherwise no pressure, however great, is able to convert the gas into a liquid ; 
and (2) in order to convert a solid body into a liquid, the pressure must be 
above a certain definite point, which the author proposes to call the critical 
pressure of the substance : otherwise no temperature, however high, is able to 
convert the substance into a liquid. If the second condition deduced from the 
above is correct, it follows that, when the required temperature is reached, the 
conversion of the body into a liquid only depends on the pressure applied to it; 
so that if we keep the pressure on a substance below this critical pressure, no 
amount of heat will convert it into a liquid. In this case the solid substance 
passes directly into a gaseous condition ; in other words, it sublimes without 
melting. When this conclusion is arrived at, it is easily seen that, if this 
representation is a correct one, it is possible to keep ice in the form of 
ice at temperatures which lie far above the melting point. After a great 
number of experiments which proved to be failures, Carnelley had the good 
fortune to attain the desired result, and, to keep ice at so high a temperature 
that it was impossible to touch the tube which held it without burning one- 
self. This result has been obtained many times, and with great ease ; and 
not only this, but in one case a small quantity of water was frozen in a glass 
vessel, which was so hot that it could not be touched by the hand without 
burning it. I have kept, he writes, a quantity of ice a considerable time at 
temperatures which lie far above the ordinary boiling point, and then it slowly 
sublimed away without first melting. These results are arrived at when the 
pressure is kept down below 4 6 mm. of mercury, that is to say, below the 
tension of aqueous vapour at the freezing point of water. Other substances 
show the same properties. The most remarkable is mercury chloride, because 
in this case the pressure need only be reduced to 420 cc. If the pressure be 
allowed to rise higher, the chloride immediately becomes liquid. In the case 
of water, there are certain details of manipulation which require attention, 
and which will be communicated in a second paper. 
Guthrie's Cryohydrates . — When investigating the freezing-point of dif- 
ferent salt solutions, Guthrie discovered some peculiar solid products to 
which he gave the name of cryohydrates. They are definite hydrates of the 
salts in question, which have a fixed solidifying and melting point, and in 
