1887.] Mr J. J. Coleman on a New Diffusiometer. 
375 
densities, but the conclusion lie came to, that under the conditions 
of his experiments the quantities diffused are directly as the times of 
diffusion , is easily and elegantly shown by using concentrated acids 
or alkalies instead of common salt. 
Thus taking a tube 9 millimetres in diameter and 20 ’5 centimetres 
long, and cementing it into a reservoir, which in shape may con- 
veniently be that of the reservoir of a glass spirit-lamp, holding 350 
grammes of hydrochloric acid of 1 T7 sp. gr., and immersing the whole 
in a jar 25 centimetres high and 12 cm. diameter, kept at a uniform 
temperature of 16° C., containing 2000 c.c. of water, and changing 
the water every two or three days, and commencing after the 21st 
day to estimate the acid diffused, it was found to be very uniform, 
viz. : — 
Milligrams. 
9 9 '9 from 21st to 24th day, 
98*4 ,, 24th to 27th ,, 
103-2 „ 27th to 30th „ 
98*4 ,, 30th to 33rd ,, 
Milligrams. 
33- 3 per day. 
32-8 
34- 4 
32-8 
55 
55 
55 
Average, 33 - 3 per day. 
The use of such modern indicators as phenolphthalein and 
methyl orange has rendered the end reaction of a volumetric pro- 
cess so delicate, that no difficulty is experienced in measuring such 
small quantities as one part of acid in 20,000 of water, which were 
about the conditions of these experiments. 
Turning attention now to what happens in a diffusion tube before 
“ dynamic equilibrium ” is established, which indeed is typical of all 
eases in which the diffused column is constantly being elongated by 
ascent of fresh particles from the bottom of a tube of constant 
diameter, I have devised a piece of apparatus which renders this 
motion visible to the eye, and which mathematical considerations 
developed by physicists indicate, should he as the square root of the 
times of diffusion. 
The principle upon which the apparatus is constructed is as 
follows : — 
If a glass jar is nearly filled with very dilute acid coloured red 
with methyl orange (sodium methyl-amido-azo-benzene-sulphonate), 
and a solution of caustic soda or potash is run by a fine pipette to the 
bottom of the jar underneath the acid, the alkali diffuses and changes 
