Institute of France. 459 
we allow condensed air to escape: he has also proved that at 
all times of the year it is sufficient that the air should be c n- 
densed in a two-fold degree, in order to obtain ice, and he 
thinks that we may procure it easily in warm countries by con- 
densing the air by means of a fall of water. 
We may, by employing bodies more evaporable than water, 
attain degrees of cold truly astonishing, and freeze not only 
quicksilver but the purest alcohol. M. Configliacchi of Pavia 
has frozen mercury by the evaporation of water alone. It was 
thought that this pressure of the air, the influence of which is 
so powerful in retarding the evaporation of liquids, also retarded 
the solution of salts, or, what comes to the same thing, accele- 
rated their crystallization when they were dissolved ; and in fact 
a saturated solution of Glauber salts, or sulphate of soda, which 
preserves its fluidity when itis cooled in vacuo, also crystallizes 
when we admit air toit. But M. Gay-Lussac says that this dues not 
happen to all salts indiscriminately ; and even with respect to the 
sulphate of soda, the phznomenon is not occasioned by the cir- 
cumstance alleged. When we intercept the contact of the 
air, by a stratum of oil for instance, the crystallization is re~ 
tarded as when we suppress its pressure by making a vacuum ; 
whereas, on the contrary, the pressure of a column of mercury 
in no respect accelerates this crystallization. A solution which 
passes through mercury, from which the air has been driven by 
ebullition, does not crystallize, and if it passes through common 
mercury it coagulates instantly. Agitation, the introduction of 
a small crystal, and many other causes, produce the crystalliza- 
tion, whatever be the degree of pressure. Thus M. Gay-Lussac 
concludes, that it is not by its pressure that the air diminishes 
the dissolving power of water. He affirms also, that it is not 
by absorbing air that water loses this power; but he thinks that 
it is a phenomenon more or less analogous to that of pure wa- 
ter, which, as is well known, remains fluid at some degrees below 
its real freezing point, when we prevent it from being shaken ; 
but it freezes the instant we give it the least shake. 
The most evident source of heat in the world is the sun’s 
rays. But it has been long remarked that these rays divided by 
the prism do not all give an equal heat; and M. Herschel, the 
celebrated astronomer, ascertained some years since that their 
power of heating went on augmenting from the violet to the 
red: he even ascertained that outside of the spectrum there 
were rays which, without being luminous, possessed a heating 
property more powerful than that of the red rays. Messrs. 
Ritter, Beckmann, and Wollaston announced soon afterwards 
that the power of the luminous rays to produce certain chemical 
changes 
