On Water and Air. 
1880.] 
249 
became covered with cloud, though while the air was passing 
over the plains of Lombardy it was perfectly clear. 
Mr. Cottrell has, by means of a pressure syringe, com- 
pressed a quantity of air into this iron box (v, Fig. 25), and 
I want to show you now that, simply by being allowed to 
expand, the air in this box will chill itself. You see that 
when I chill the face of this instrument which is connected 
with the galvanometer, the red end of the needle goes away 
from me. The smallest contadt with the chilled blade of a 
knife causes the red end of the needle to move in that way. 
I will warm the instrument a little, so as to bring the needle 
on the side which indicates heat. Now, what I want you 
to understand is this : — Here I have compressed air. At 
the present moment that air is warmer than the air in this 
room. I will now cause the air to expand by simply turning 
the stopcock and allowing it to issue. It will force itself 
out, and a chilling will be produced. The work done by the 
air in forcing itself out of this box will cause a chill in the 
air which forces the first portions of the air out. Thus the 
last portions of the air will be so chilled that you will find 
that the red end of the needle will go across zero and up on 
the other side. [The compressed air was allowed to escape 
from the iron box, v, and impinge upon the face of the 
thermo-eleftric pile, p, connected to a galvanometer.] Al- 
though the air when compressed was warm, yet it has 
become chilled by its own expansion, and has chilled the 
pile. You see that the needle comes down from the direc- 
tion of heat, and I have no doubt that it will cross zero and 
go up considerably upon the other side, proving the chilling 
