BATH CHEMISTS’ ASSOCIATION. 
505 
would be readily understood to be in direct proportion to the height of the columns of 
liquid above it; that is, if at one foot the pressure equalled one pound, at two feet it would 
equal two pounds, at three feet three pounds, and at four feet four pounds. But con¬ 
vert that spot into an aperture, and allow the liquid to flow, and Torricelli had shown 
the rate of flow would not increase in the same direct proportion to the height of the 
column, but in a proportion which coincided with the square root of the height of the 
column. If, therefore, the flow at one foot equalled one pint, the rate would not be two 
pints, until the height of the column had increased to four feet, nor to three pints until 
the column were nine feet high; the square roots of four and nine being respectively two 
and three. This was Torricelli’s law enunciated, the lecturer believed, simply in con¬ 
nection with the relation which the rate of flow bore to the height of the column, but 
Dr. Attfield was at the present moment at work upon the subject, and his observations 
had led him to include Torricelli’s law in one of larger scope, which might possibly be 
expressed somewhat thus, “ The efflux of fluids from apertures is in proportion to the 
square roots of the powers operative/.” 
Dr. Attfield had not yet publicly announced his views, but the lecturer stated he had 
the Doctor’s permission to allude to them as he had done. He added, they appeared to 
him to be theoretically so sound, he had little doubt of their being proved true. 
He then continued -—Having now' refreshed your memories, Gentlemen, upon these 
general matters, and indicated a few points which I wish to be especially borne in mind, 
I turn to the immediate subject of my lecture, “Certain Physical Aids to Filtration.” 
If the process of filtration were really analogous, as some have appeared to think, to 
the separation of fish from the water in which they swim by means of a net, it would 
present so few difficulties that we should never need to think about aiding it in any way. 
But I deem the word filtration to mean something essentially different from “straining,” 
so essential as to involve fresh elements of force. For instance, that form of attraction 
which we have just been considering, called adhesion, does not, in the smallest degree, 
influence the process of catching fish in a net; but should any one doubt its influence in 
filtration, let him try the difference between the effort to pass an ounce of mercury 
through a filter of blotting-paper and an ounce of water. In the former case there 
being no adhesive attraction between the liquid and the filtering medium, no transmis¬ 
sion of the liquid takes place, whilst in the other case the adhesive attraction, as we have 
seen, is considerable; sufficient, indeed, to overcome that of gravitation, and hence the 
transmission of the liquid is comparatively easy. There appears, then, to be a funda¬ 
mental difference between filtration and straining; but, be that as it may, we all know 
straining to be a light and easy matter, whilst, under the most favourable circumstances, 
filtration is heavy and slow. The cohesive force which holds the particles of the liquid 
together has to be overcome; the particles have to be separated very minutely, so as to 
enter the small channels of the medium, and having arrived at its under surface must 
there wait till the accumulated gravity of many particles at last compels them to leave 
the under surface of the medium, and drop into the reservoir. 
The operation is thus necessarily slow. Practically it is even more prolonged, for we 
never filter liquids already bright, but only those from which we wish to separate 
minutely attenuated solid matter. The difficulty in such cases increases each moment 
of the operation, for the deposited solid matter renders still smaller the orifices in the 
medium through which the pure liquid has to pass, and thus adds something to the 
amount of force resisting the transmission, until at last the patience of the operator is 
fairly exhausted. 
Now it is under such circumstances, and for the conservation of this great quality 
“ patience,” that various “ aids ” to filtration have been proposed, the chief of which have 
aimed at the acceleration of the process, by means either of hydrostatic or of aerostatic 
pressure. I will not trouble you with the enumeration of all the forms of apparatus 
designed to this end, but may just mention that upon the former principle instruments 
have been constructed in which the under surface of the box already alluded to, modified 
in shape, has been converted into the filtering medium, and by continuing the tube to 
the height of a few feet, and maintaining it full of the liquid to be filtered, a great sta¬ 
tical pressure has been obtained upon the medium equal, as you will remember, to the 
weight in the tube, multiplied by the number of times the area of the medium exceeded 
the bore of the tube. Aerostatic pressure has also been produced by instruments which 
contrive that the liquid to be filtered be placed in a closed vessel above the medium, 
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