296 M. Carré on the Production of Low Temperatures. 
this apparatus through an S tube; the products pass first through 
a cooler which condenses the water, and then through a washer 
of a special form. In this way pure and inodorous gas is ob- 
tained, and a solution of sulphurous acid, which may be changed 
either into sulphite or hyposulphite of soda, or may be used in 
the sulphuric acid chambers. The expense ‘of oxygen prepared 
by this plan is very small; for the method consists essentially i in 
abstracting oxygen from the atmosphere. Hvenif the sulphurous 
acid were not utilized, sulphuric acid would still be the cheapest 
source of oxygen, cheaper even than binoxide of manganese. 
M. Carré has applied* the great cold produced by the evapo- 
ration of condensed ammoniacal gas to the production of low 
degrees of temperature. 
The apparatus he uses consists of two ordinary cylindrical 
metal receivers connected by a tube. One of these is four times 
the size of the other, and is filled to three-quarters its capacity 
with the strongest solution of ammonia. At the time of closing 
the vessel, care is taken to expel all air. The largest vessel is 
placed over the fire, the smaller being immersed in cold water. 
The solution is heated to 130° or 140°, the temperature being 
indicated by a thermometer fitted in the larger vessel. At this 
point nearly all the ammonia is expelled from the solution, and 
liquefies in the second retort. When the separation is complete, 
the larger vessel is cooled down: the reabsorption of the liquefied 
gas commences immediately, and its volatilization produces a 
degree of cold which readily freezes the water surrounding it. 
The temperature sinks to —40°; and M. Balard, who tried the 
experiment at the Collége de France, was able to solidify mercury. 
Besides this apparatus, M. Carré has devised another form of 
it, which is continuous in its action, but otherwise depends on 
the same principle. 
M. Leroux, of the Ecole Polytechnique, has made some deter- 
minations of the refractive indices of vapours at high tempera- 
tures, by means of an apparatus constructed for that purpose, on 
which M. Babinet+ has reported to the Academy. 
M. Dulong had found that the refractive index of oxygen 
was 1:000278 ; of hydrogen, 1:000188; of nitrogen, 1-000300 ; 
and of chlorine, 1:000772, that of air being 1:000294. M. 
Leroux has found that the refractive indices of the vapours of 
the following substances, saturated at the ordimary pressure, are 
respectively— 
* Comptes Rendus, December 24, 1860, 
+ Ibid. November 26, 1860. 
