38'2 
PROFESSOR GRAHAM ON THE MOTION OF GASES. 
in both cases. This is in accordance with the general observation, that transpiration 
is promoted by increase of density. The theoretical density of chloride of methyl is 
25'25 to hydrogen 1 and oxygen 16. 
16. Water. 
Although great care was always taken to dry air when transpired, as well as other 
gases, in all experiments, still it does not appear that the rate of air is much affected by 
the presence of aqueous vapour unless the latter is present in considerable proportion. 
The times observed by capillary K, 8’75 inches in length, into a vacuum, were for 
air dried by chloride of calcium 1008 seconds, and for air drawn afterwards directly 
from the atmosphere, of which the temperature was 60° and the dew-point 32°, 1006 
and 1006 seconds. So small a difference may be due to accidental causes. 
With dry air at 60°, the times with the same capillary were, upon another occasion, 
1021 and 1021 seconds; and with air of 60° temperature, but containing aqueous 
vapour with the dew-point at 38°, 1018 and 1017 seconds. 
In other experiments, the presence of aqueous vapour appeared to occasion a 
sensible retardation in the time of air. The transpiration was made into a vacuum by 
the capillary M, 52'5 inches in length ; the temperature of the capillary being main- 
tained at 58°‘5, and the barometer varying from 29’798 to 29‘832 inches. The air 
was charged with vapour by passing through a tube filled with cotton wick, which 
had been previously moistened with dilute sulphuric acid of different strengths. The 
time of dry air was 1115, 1115 seconds; of air carried over the fourth hydrate of 
sulphuric acid (HO .S 03 -f 3 H 0 ), 1117, 1117 seconds ; of air passed over the eighth 
hydrate (HO . SO 3 -I- 7 HO), 1120 and 1121 seconds ; of air passed over the eighteenth 
hydrate (HO . 803 -!- 17 HO), 1122 , 1122 and 1121 seconds. Here we observe in the 
dampest air a slight but sensible increase of the air time, not exceeding 7 seconds. 
But on repeating the experiment immediately afterwards with dry air, the time was 
1120 and 1119 seconds, or within two seconds of the immediately preceding obser- 
vations with moist air. Indeed the transpiration of moist air appears to produce a 
slight but sensible retardation of a persistent character, probably from the condensa- 
tion of a film of moisture on the inner surface of the capillary, which is not imme- 
diately removed by the subsequent passage of dry air. 
With the same capillary, thermometer 57° and barometer 30T36 to 30'078 inches, 
dry air was transpired in 1089, 1089 seconds ; dry hydrogen in 532 and 532 seconds ; 
air saturated with aqueous vapour at 57°‘5 in 1098, 1098 seconds ; hydrogen satu- 
rated with aqueous vapour at the same temperature, in 548 and 548 seconds ; and 
lastly, dry air, first in 1106 seconds, and afterwards in 1084 and 1085 seconds. Here 
the damp air is less transpirable, volume for volume, than dry air by 9 seconds. Also, 
dry air immediately following the damp air does not recover its usual transpirability 
in the first experiment. 
These experiments upon damp and dry air seem to indicate that the transpiration 
