16 Scientifie Proceedings, Royal Dublin Society. 
that “the air is said to have been humid, and yet, when reduced 
to a pressure of one-sixth of an atmosphere, to have contained 
‘less than one-sixth of the vapour necessary to saturate the space.’ 
But if the air was anywhere near saturation at the ordinary pres- 
sure, it must have been supersaturated when reduced to a pressure 
of 5 inches—a fact which was pertectly well known to Tyndall, 
since he has described it on page 46 of the same work. I can 
only reconcile these statements by supposing that either Tyndall 
inadvertently overlooked the increase of relative humidity in air 
at reduced pressure, when writing this passage, or else that the 
description of the air as ‘ humid’ is very misleading.”* 
The page 46 referred to by Very is in the American edition 
of “Heat a Mode of Motion,” which I have not been able 
to consult; and I am therefore quite at a loss to imagine what 
process Tyndall could have described which could justify so 
amazing a criticism. Pushed to its logical extreme Very’s argu- 
ment implies that water cannot evaporate in a vacuum. Probably 
the point which Very refers to is the chilling of air by its own 
rarefaction, which Tyndall thus describes :—‘“ On the plate of the 
air-pump is placed a large glass receiver filled with the air of this 
room. ‘This air, and indeed all air, unless it be dried artificially, 
contains a quantity of aqueous vapour, which, as vapour, is per- 
feetly invisible. A certain temperature ts requisite to maintain the 
vapour in the invisible state; and if the air be chilled so as to 
bring it below this temperature, the vapour will instantly condense 
and form a visible cloud. . . . A very few strokes of the pump 
suffice to precipitate the vapour. . . . When the air is allowed to 
re-enter the vessel, i is heated, the cloud melts away, and the 
perfect transparency of the air within the receiver is restored.’” 
The point of the quotation will be found in the words I have 
ventured to italicise. Temperature, indeed, is all-important ; 
pressure of no importance at all, excepting that the temperature 
and pressure will at first, in any such experiment, fall together. 
Tyndall had very fully described what he called “‘ dynamic radia- 
tion and absorption,” as resulting from dynamic heating and 
1. W. Very, ‘‘ Atmospheric Radiation,’* 1900, p. 81. 
* J. Tyndall, ‘‘ Heat a Mode of Motion,’’ 1880, p. 16. 
