384 
ME. T. GEAHAM ON LIQUID TEANSPIEATION 
found advantageous to operate at a fixed temperature, which is somewhat elevated. A 
large number of substances are liquid at 100 °, of which the transpiration-time could be 
easily obtained. 
In hydrated substances transpiration also affords a manifestation of definite combi- 
nation at once striking and precise. I need only refer to the manner in which the 
“constitutional” hydrate of sulphuric acid SHO4+HO, of acetic acid C 4 H 4 O 4 + 2 HO, 
of nitric acid NHOg+SHO, and of alcohol C 4 Hg 02 + 6 H 0 is each indicated by its 
maximum transpiration-time. The indication of the alcohol-hydrate is particularly 
distinct, although that hydrate must be a comparatively feeble compound. Indeed 
the extent to which transpiration is affected by the annexation of constitutional water 
appears to be by no means in proportion to the intensity of combination. 
The increased resistance to transpiration observed in these definite hydrates may be 
connected with their larger molecules. But another speculative view of the retardation 
can he suggested, in which the phenomenon is referred to a physical agency. When 
one of these definite hydrates, say the 6 -hydrate of alcohol, is being forced through the 
capillary, it may be imagined that a small portion of the hydrated compound is mole- 
cularily decomposed by the friction. A certain portion of the impelling force would 
thereby be lost, being converted into the latent heat which alcohol and water require 
to assume when separated from each other, and the transpiration be consequently 
retarded ; for as alcohol and water evolve heat on combining, so they must absorb 
heat when their union is dissolved by any cause. But the change of tempei uture repre- 
senting the lost force appears to be too small to be rendered sensible to observation. It 
would be capable of raising the temperature of the transpired liquid not more than 
about one forty-thu’d part of a degree, according to an accurate estimate for which I am 
indebted to Professor Stokes. In consequence of this circumstance the physical hypo- 
thesis now suggested has neither been verified nor disproved. 
To this paper are appended two series of observations made on transpiration at 
different temperatures, the first series being the transpiration of water, and the second 
that of absolute alcohol. Each series of experiments is repeated with two capillaiy 
tubes, one having nearly double the resistance of the other. The numbers from the 
two capillaries exhibit a fair amount of agreement. The times given are those 
actually observed, no correction being made for the small variation of the capillary in 
diameter at different temperatures. 
The dimensions of Capillary D were as follows: — Capacity of bulb, 4T35 cub. cent.; 
length of capillary tube, 37‘5 millims. ; diameter of bore, 0T0325 millim. Time of 
passage of water, at 20°, under pressure of one atmosphere, 470 seconds. 
The dimensions of Capillary E were as follows: — Capacity of bulb, 3'725 cub. cent.; 
length of capillary, 53 millims. ; diameter of bore, 0-0858 millim. Time of passage of 
water, at 20°, under pressure of one atmosphere, 913 seconds. 
