580 
PROFESSOR GRAHAM ON THE MOTION OF GASES. 
manner when the gas passes through an aperture in a plate of no sensible thickness, 
for when the opening becomes a tube, however short, the effluent gas meets a new 
resistance which varies in the different gases according to an entirely different law 
from their rates of effusion, namely, the resistance of transpiration. The deviation 
is most considerable in hydrogen, which rapidly loses velocity if carried through a 
tubular opening, when compared with air. This was illustrated by experiments made 
upon the glass jet of the former observations ; which was operated upon in four dif- 
ferent conditions as to length. The point had been drawn out rather long at first, so 
that it admitted of portions of 0*2 inch, 0T inch, and 0’07 inch, being broken off 
successively before it was reduced to the form of a blunt cone, which it had when 
used in the experiments already detailed. Air and hydrogen were effused from this 
jet into the exhausted receiver till the mercurial gauge fell from 28 to 4 inches, with 
the jet in the different states described. 
When the glass jet w r as of greatest length, the time of air was 335 and 337 seconds 
in two experiments, and of hydrogen 120 seconds in two experiments; which give 
2*800 as the velocity of effusion of hydrogen. 
After the first portion was broken from the point, by which of course the aperture 
was enlarged, the time of air was 175 seconds in two experiments, of hydrogen 55 
and 56 seconds ; giving 3*153 for the effusive velocity of hydrogen. 
After the second abridgement in its length, the time of passage of air by the jet was 
110 seconds in two experiments, of hydrogen 33, 32 and 33 seconds in three experi- 
ments ; giving 3*33 for the velocity of hydrogen. 
When still further reduced in length, a larger jar being used as the vacuous re- 
ceiver, the time of air was in two experiments 408 and 410 seconds ; of hydrogen in 
three experiments, 122, 120 and 122 seconds, giving 3*38 for the velocity of hydrogen. 
Thus, as the jet was progressively shortened, the relative velocity of the passage of 
hydrogen continually rose, passing through the numbers 2*8, 3*153, 3*33 and 3*38. 
By reversing the direction of the stream of gas through the aperture in its last con- 
dition, the effect of friction was still further diminished, and the velocity of hydrogen 
raised to 3*61, as in the experiments previously recorded, which were made with this 
jet in an inverted position. It may be fairly presumed, therefore, that if the length 
of the tube or thickness of the plate containing the aperture was still further di- 
minished, the effusive velocity of hydrogen, compared with air, would be increased, 
and approximate more nearly to 3*80, the theoretical number. 
The tubularity of the opening quickens, on the contrary, the passage of carbonic 
acid and nitrous oxide in reference to air ; for these gases are more transpirable than 
air, although less effusive : hence their observed time of effusion is always sensibly 
less than their calculated time. 
