376 Proceedings of Royal Society of Edinburgh. [july 15 , 
the colour of the methyl red to bright yellow, the line of demarcation 
being as strongly defined as that of oil floating upon water, and this 
even if the diffusion is carried on for thirty -five or forty days. 
The caustic alkali solution is most advantageously placed in an 
open-mouthed vessel of such capacity that a barometer tube, 1 2 mm. 
to 15 mm. inside diameter, containing the reddened acid can be 
overturned therein with its mouth downwards, the acid being kept 
in the tube by means of an india-rubber disc, attached by three 
platinum wires to a cork sliding on the tube, which thus acts as a 
valve to be thrust down when the time for starting diffusion arrives. 
The tubes I prefer are about 12 "5 mm. diameter and 600 mm. long, 
accurately graduated in millimetres. 
The drawing herewith illustrates the construction of this instru- 
ment or “liquid diffusiometer,” surrounded byja glass bell’, jar to 
prevent its being affected in temperature by air currents. 
