EADIATION OF HEAT BY OASEOTJS MATTEE. 
91 
salt used to stop my tube. The substance is well known to be very hygroscopic, and 
during the last three years the knowledge of this fact has rendered me careful to remove 
my polished plates every evening from the apparatus, and to keep them in perfectly dry 
air. Still, when it is remembered that the air on entering the tube is raised in tempe- 
rature, and thus enabled to maintain a greater amount of vapour, and that the tube and 
plates of rock-salt form the channel for a flux of heat from the radiating source, the 
likelihood of precipitation occurring will seem but small. On examining the plates 
after the undried air of the laboratory was experimented with, no trace of precipitated 
moisture was observed upon their surfaces. 
But to place the matter beyond all doubt, I abolished the plates of rock-salt altoge- 
ther, and operated thus : — An india-rubber bag (B) was filled with air, and to its nozzle 
a T-piece, with the cocks Q Q', was attached. The cock Q' was connected with two 
tubes, U' U', each of which was filled with fragments of glass moistened with distilled 
water. The cock Q was connected with the tubes U U, each of which was filled with 
fragments of glass moistened by sulphuric acid. The other ends of these two series of 
tubes were connected with the cocks O O', and from the T-piece between these cocks 
a tube led to the end E' of the open experimental tube T. The cock A at the other 
end of the experimental tube was placed in connexion with an air-pump. The pile P, 
N 2 
