506 
BATII chemists’ association. 
and that air be condensed upon it by means of a syringe. The same practical effect 
has been sought by an arrangement which exhausts the vessel beneath the medium of 
portion of its air, by which means a differential pressure upon the upper surface has been 
brought about. The exhausting apparatus in these cases has been a pump or syringe, 
to be worked by mechanical agency. 
Now, though many of these instruments have been very ingeniously conceived, they 
have been all open to certain practical inconveniences. In the hydrostatic apparatus 
the force employed was so great that it was necessary from the first to construct the 
medium of proportionately strong and close texture, hence the resulting advantage was 
proportionately decreased, and in the contrivances depending for their result upon aero¬ 
static pressure the necessity for the almost constant use of the pump was almost as great 
a trial of the patience as was the original dilemma. 
In the instrument I am about to describe I have endeavoured to reap all the aid which 
the scientific disposal of natural forces could bestow, and at the same time to absolve the 
operator from all but a minimum of attention and labour. 
The apparatus consists of a cylindrical vessel a a B B of pure tin, slightly smaller in 
diameter at the bottom than at the top. The smaller end is provided with a ledge, C, 
from which the bottom slopes to a central spout, D. A perforated plate of tin, E, 
strengthened at the under side by cross pieces of the same metal, rests upon the ledge. 
A ring of tin, F, a quarter of an inch thick, strengthened with cross pieces, is also pro¬ 
vided. Upon this ring the filtering medium, flannel, calico, felt, etc., is stretched, and 
the whole is placed (medium downwards) upon the perforated plate. The ring should 
be so contrived as to press the filtering-medium tightly against the sides of the vessel, 
and yet just to rest upon the perforated plate. A small hole, G, is drilled horizontallv 
through the ledge, opening into the instrument close under the perforated plate. A tube 
is then provided, of any length, from one foot to thirty, and at one extremity is reversed 
upon itself twice. This tube may be of various materials, but on the whole vulcanized 
india-rubber is the most convenient. In that case the bent piece at the end should be 
of some solid substance, such as glass. 
To put the apparatus into action, stretch the filtering-material over the ring, F, and 
press it into its place upon the perforated plate, E. Attach the tube to the spout, D, 
and close the extremity, H, with a cork. Pour in the liquid to be filtered. A portion 
soon passes through the medium and fills the tube, the air escaping through the small 
hole, G. As soon as the liquid begins to escape at this hole, stop it with a little wax ; 
remove the cork from H, and the action commences. 
The dimensions of the instrument and the material of its construction will, of course, 
