[ 4Si ] 
only arile from a difference in the caufe that ads 
upon them. 
9 5. We may form our ideas of the manner in 
which the invifible humor diftributes itfelf; from 
that in which all kinds of vifible vapours are diff 
fufed. We fee them feparate, re-unite, flv off from 
certain places, rulh into others, and in fhort yield 
to every impreffion of the air. The motion pecu- 
liar to their own particles, which I look upon 
as the caufe of their elafticity *, is not fufficiently ra- 
pid, and the vapours themfelves are too thick to 
overcome always the contrary motion of the air. 
This,I believe, is what conffitutes the chief difference 
between vapours, and the igneous fluid, as far as 
relates to the power of putting themfelves into a. 
Hate of equilibrium in the air, which is moving. 
The current of air carried towards a chimney which 
has fire in it, frees the room from fmoke, and is. 
but a very flight impediment to the diffufion of the 
heat through it. 
9 6 . Though the invifible vapours by reafon of 
their exceflive thinnefs are more capable of being 
put in equilibrium in the air than the vifible ones, 
they are very far from having this property in as 
great a degree as the- heat. Which leads me to 
think, that part of the difference obferved between 
my hygrometers, even before funrife, may have 
been owing to the unequal difhibution of the hu- 
mor, though the two inflruments were only at the 
diflance of a foot from each other, without the in- 
terpolation of any folid body. 
* 1 he fyftem I adopted on that point may be found in my 
work upon the Modifications of the atmojphere. 
Vol. LXIII. N mn 97. I 
