372 
TRANSACTIONS OF THE PHARMACEUTICAL SOCIETY. 
when atmospheric pressure is removed. The practical means, I would propose, 
of getting rid of such accumulations, I have already described, namely, in the 
introduction of a small glass reservoir between the upper surface of the filtering 
medium and the surface of the water in the vessel in which the medium is 
placed. A small stopcock in the top of such reservoir would allow of the expul¬ 
sion of any accumulated air on merely closing the orifices of outflow. For example 
the carbon block and casing shown in the accompanying 
woodcut of the “cistern filter ” of Messrs. Atkins might 
have such a reservoir placed above the centre of the 
block, communicating by a glass tube and cork with the 
central cavity of the block. Whether or not an increased 
flow from such a filter is desirable, and would be more 
than equivalent to the extra complexity and cost of the 
instrument, I will not now determine; nor can I state how far such a flow through 
the carbon block might in time impair its porosity. I merely throw out the hint. 
Fifthly, since writing this paper, and only a few hours before its delivery, Mr. 
Schacht has told me that his original idea in devising the form of filter he de¬ 
scribed at Birmingham, was simply the production of an aerostatic instrument, 
the hole in the upper part of the chamber beneath the filtering medium being- 
placed there to admit of the long tube being conveniently filled, and not to en¬ 
sure the removal of every trace of air from that chamber. On examining the 
filter he brought to me, I at once saw that all the air was not removed, that 
sufficient remained to interfere with though not to destroy the continuity of the 
column. I would, therefore, more distinctly than in the last paragraph, propose 
an improvement in his filter. I would suggest the blocking up of the small 
side-hole in his instrument altogether, and the adoption of a narrow channel of 
communication from below to above the medium, terminating in a glass flask 
and stopcock as already described. The plane of the perforated metal plate on 
which the medium rests, might be slightly inclined, and the small channel be 
then at the side rather than in the centre of the medium. 
I am indebted to Mr. Watts for the trouble he has taken in assisting me in 
the performance of the foregoing experiments. 
The Chairman said, as the usual hour for adjournment was already passed, 
it would be hardly be possible, on that evening,To enter upon the discussion 
of the paper that had just been read. He observed, however, that Mr. 
Schacht, whose apparatus had been referred to, was present, and the meeting 
would no doubt be glad to hear his explanation of his filter, unless he could 
make it convenient to attend on another occasion. 
Mr. Schacht said he did not think he should be able to attend the next 
meeting, and, as it seemed to be the wish of the meeting that he should say 
something about his own apparatus, he would endeavour, in a few words, to 
explain its origin, and to point out the advantages he ascribed to it. Having 
occasion to devise some means for promoting the filtration of oils and sub¬ 
stances, which in the ordinary way pass through a filter with difficulty, he 
endeavoured to render the pressure of the atmosphere available by introdu¬ 
cing a column of liquid beneath the filter and in contact with it, the effect of 
which was to remove apart of the upward pressure of the atmosphere against 
the bed of the filter, and thus to give a preponderance to the downward pres¬ 
sure. This preponderance, or differential pressure as he had called it, de¬ 
pended upon the length of the column of liquid beneath the filter. The ar¬ 
rangement was a simple one, and he found it to be efficient. The principle 
upon which its efficacy depended was, he thought, sufficiently obvious, and he 
did not think Dr. Attfield had represented its action very correctly. In prac¬ 
tical operations in filtration, where the assistance of atmospheric or other 
