384 
PROFESSOR GRAHAM ON THE MOTION OF GASES. 
A good deal of light could be obtained, I believe, upon the composition and value 
of coal-gas by a combination of effusion and transpiration experiments. Great 
density, which would be indicated by slow effusion, is always valuable, unless when 
occasioned by air, carbonic oxide or carbonic acid, which gases exclusively make the 
transpiration slow ; so that slow effusion with rapid transpiration would mark the 
coal-gas of superior quality. 
III. TRANSPIRATION OF AIR OF DIFFERENT DENSITIES OR ELASTICITIES. 
A series of observations on air varying in density from 0‘5 to 2 atmospheres, made 
with the long 20-feet capillary E in my former paper, appeared to establish the con- 
clusion that “ for equal volumes of air of different densities, the times of transpiration 
are inversely as the densities.” The law of Effusion, or flow of air into a vacuum by 
an aperture in a plate, is entirely different ; equal volumes of air of all densities 
passing in equal times. 
With the short capillary K, 8*75 inches in length, the result was now found to be 
materially different. Air in three different states of rarefaction was drawn into a sus- 
tained vacuum from a globular receiver of which the capacity was 56’5 cubic inches, 
standing over water. To command the desired density of the air in the globular 
receiver, the little system of the latter and the basin of water in which it stood was 
retained within a large air-pump receiver, the atmosphere of which was adjusted to 
the requisite pressure. Thermometer 62°, external barometer from 29'984 to 29*936 
inches. 
Transpiration of equal volumes of Air. 
Density or elasticity. 
Time in seconds. 
Experiment I. 
Experiment II. 
1 atmosphere 
2172 
2173 
0'75 atmosphere 
2948 
2946 
0*5 atmosphere 
5292 
5288 
It will be observed that the time 5292 seconds for air of 0*5 density is considerably 
more than double 2172 seconds, the time for air of 1 density. 
With compressed air, varying in density from 1 to 2*5 atmospheres, the deviation 
from the law was equally conspicuous ; the times of transpiration of equal volumes 
at 1, 1*25, 1*5, 1*75, 2 and 2*5 atmospheres, being in the ratio of 1, 0*8625, 0*7553, 
0*6834 and 0*5519, instead of 1, 0*8, 0*6666, 0*5714, 0*5 and 0*4. 
On operating, however, with the long capillary M, 52*5 inches in length, and of 
great resistance, results were again obtained in strict accordance with the law. The 
air was drawn from a metallic digester provided with a gauge barometer, in which 
it was preserved of a constant elasticity ; this digester itself being supplied from a 
second similar digester, in which the air was in a state of still higher compression. 
