BATH CHEMISTS’ ASSOCIATION. 
507 
vary with the purpose for which it is to be employed. That which I have had made for my 
own use is of pure tin. It is ten inches high, nine inches in diameter at the larger, and 
eight inches at the smaller end. The ledge is one-third of an inch deep, and the tube 
of india-rubber is four feet long. I find this elastic tube very convenient, for, as the 
necessity for pressure is not so urgent at the commencement of the process as subse¬ 
quently, when the filtering-medium has become choked, the operation can be commenced 
with efficient result by bringing the receiver to within a few inches of the bottom of the 
apparatus, and the column can be gradually lengthened and the effect proportionately 
increased by simply lowering the receiver, to each new position of which the elastic tube 
readily accommodates itself. 
With this instrument I have filtered to perfect brightness eight gallons of turbid solu¬ 
tion within the hour, and have had the satisfaction of seeing the last ounce pass through 
almost as quickly as the first. 
This is my description of the instrument originally published in the Transactions of 
the Pharmaceutical Conference, and to confirm its practical utility I venture to quote 
Dr. Attfield’s experiments with my own instrument. He found that when the orifice of 
the delivery-tube was raised to various distances from the upper surface of the liquid in 
the apparatus, the level of which was kept constant, the results were :— 
At 1 foot 31 oz. in a given time. 
„ 2 feet 57 oz. in the same time. 
o ro 
,, O ,, ,, ,, ,, 
55 55 87 ,, 55 . 55 
clearly showing, that by simply lowering the orifice of the delivery-tube (that is length¬ 
ening the column) from one foot to two, three, and four feet, we are eliciting the mani¬ 
festation of some force of very considerable power. As some of my scientific friends do 
not view the cause of this result quite as I do myself, I sh a n trouble } r ou with m 
version of the phenomena and their explanation. ^ 
a a a is supposed to represent an air-tight box terminating 
in a tube which opens under the surface of a trough of water. 
Into the upper surface of the box is fitted a tube supplied with a 
stopcock and opening upwards into a funnel which is kept filled 
with water. There is also an exhausting apparatus communicat¬ 
ing with the upmost part of the box. Now, if the stopcock be 
turned so"as to admit the entrance of, say, sixty drops of water 
per minute, that quantity will drop through the apparatus 
into the trough. The pressure of the air being the same both 
within and without the apparatus, the level of the water at 
point B is undisturbed. If the syringe be now set in motion, 
a portion of the air is withdrawn from within, and external 
atmospheric pressure begins to manifest itself. This pressure 
is felt by every part of the apparatus, including the stop¬ 
cock and the extreme orifice of the tube. Both these parts 
being, as it were, stopped with water, a similar kind of effect 
is attempted to be produced at both the top and the bottom of 
the instrument, and the liquid at both points is driven inwards. 
But as there is considerable resistance to its passage at the 
top and none at the bottom, it ascends freely at the latter, to 
a height depending upon the amount of air withdrawn, say to 
one foot, and, at the former, the effect is simply increased 
dropping. But this dropping, though continuous, does not 
alter the level of the water in the tube, for as many drops 
escape into the trough as enter at the stopcock. If the pump 
be now again set in motion more air is abstracted, the pres¬ 
sure from without increases, forcing the water still higher in the tube and still more 
rapidly through the stopcock. The same results, varying only in degree, would attend 
the continued abstraction of fresh portions of the air by the pump; the water would 
gradually rise to two, three, four feet in the under tube, until at last it communicated 
with the small stream issuing from the point of the stopcock. 
Now, throughout these processes, the exhausting pump has clearly been the agent to 
bring about a partial vacuum, and a consequent differential pressure of the air,—the 
2 l 2 
