250 Proceedings of Boy cd Society of Edinburgh, [march 19, 
bling a barometer tube, which was, after being inverted, immersed 
in the reservoir of the substance being diffused. 
The progress of the diffusion was registered by a sharp change of 
colour and line of division, as easily read as that of oil standing 
over water, at a point where the ascending particles became 
sufficiently numerous to neutralise the acid or alkaline condition, 
so that the instrument afforded an ocular demonstration of the 
upward march of an ascending column, which, from observations 
conducted for thirty or forty days, was found to follow with 
extreme precision the square root of the time of diffusion. 
It is obvious that the use of this instrument is limited to acid or 
alkaline liquids, unless it can be made to work with some other in- 
dicator than methyl orange. 
The neutral salts, such as chlorides and sulphates, being desirable 
subjects for investigation, it occurred to me that soluble salts of 
silver could be used as indicators for chlorides, and soluble salts of 
barium as indicators for sulphates, provided jelly be dissolved in 
the water, so as to entangle the precipitates produced. 
This expectation was very satisfactorily realised by filling the 
diffusion tube with jelly containing 5 per cent, of isinglass, and 
TU(To weight of the indicator, viz., a salt of silver or barium. 
Under such circumstances, the ascending column converts the 
transparent jelly into what looks like a rod of ivory surmounted 
with transparent jelly. 
Although the progress upward is not so mathematically exact as 
in the case of the employment of methyl orange, it certainly follows 
with wonderful precision a speed corresponding with the square root 
of the time of diffusion. Compared, however, with diffusion into 
pure water, there is a retardation, owing to the presence of the jelly. 
Gelatine combines to some extent with salts of silver, but 
there seems to be less objection to the employment of gelose or 
Japanese isinglass, 1 per cent, of which is as effectual as 5 per cent, 
of ordinary gelatine, besides which gelose does not prevent the 
precipitation of silver salts by chromates, which occurs with gelatine ; 
and this is a matter of interest, as red insoluble chromate of 
silver suspended in jelly is converted into white chloride by diffusing 
into it any soluble chloride. 
I have annexed a table herewith of experiments which [lasting 
