202 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
10° to 12°. It will be observed that these volumes correspond to a depth of liquid in 
the dialyser of O’ 4, 0’2, and O’l inch respectively. 
The time of travelling through the thickness of the parchment-paper itself may be 
obsen ed, and is worthy of remark. 
Of the quality of parchment-paper always used in these experiments, a square metre, 
when dry, weighed 67 grammes ; and when charged with water, 108’6 grammes. Taking 
the specific gravity of cellulose at 1’46, that of the lighter woods, the parchment-paper 
described will, in the humid state, have a thickness of 0’0877 millimetre, or of a 
millimetre. Wet parchment-paper so thin is highly translucent. Gelatinous starch, 
slightly coloured with blue litmus, was applied by a brush to one side of the wet parch- 
ment-paper. Immediately afterwards a drop of water, containing y^^dth part of hydro- 
chloric acid^ was applied on the point of the finger to the other (the lower) side of 
the paper. The time required by the acid to afiect the litmus, in five successive trials, 
w’as 6 seconds, 5 ’5 seconds, 6 seconds, and 5 seconds. The mean is 5 ’7 seconds, which 
is therefore the time required by hydrochloric acid, diluted already 1000 times, to travel 
a distance of 0’0877 of a millimetre, by the agency of diffusion. The temperature 
w'as 15°. 
With hydrochloric acid diluted twice as much as before (water containing 0-0005 dry 
acid), the average time of passage was 10-4 seconds, or nearly double the preceding 
time. 
Water containing -yo^dth of sulphuric aoid (an acid less rapidly diffused than hydro- 
chloric acid) reddened the litmus in 9-1 seconds, and when doubly diluted in 16-5 
seconds. 
These results are not affected, it is believed, by any sensible diffusive movement on 
the part of the litmus. The difiusion of that colouring matter, in a colloid medium, is 
so slow that it may be entirely disregarded. The acid, therefore, is not met in its way 
by the litmus, but really travels the entire distance expressed by the thickness of the 
parchment-paper. The first experiments related give a diffusive velocity, in water, to 
hydrochloric acid, already diluted one thousand times, of 0-0154 millimetre per second, 
and 0-924 millimetre in one minute. 
The few following dialytic experiments may be recorded for the sake of the practical 
points which they bring out. They were made in the smaller osmometer, with 100 cub. 
cents, of a solution containing 10 grammes of each of the various substances. The area 
of the parchment-paper septum was 0-005 square metre, and the depth of the stratum 
of fluid placed upon it 20 millimetres. The substances diffused were all crystalloids, 
with the exception of gum-arabic. 
