of Edinhurghy Session 1882-83. 
139 
fitted with platinum cone and filter in the ordinary way. To the 
stem of the funnel the glass tube bent as shown in the figure, 
and having the lower end ground obliquely, is attached by a short 
piece of black rubber tubing, the upper end of the tube being 
pushed up so as to be in actual contact with the stem of the funnel. 
The bend makes it easy to cause the end of the tube to touch the 
side of the beaker, which prevents spirting. It will be found 
convenient to keep several such tubes of different lengths ready 
made to suit larger and smaller vessels. Before commencing oj)era- 
tions it is as well to slightly moisten the rubber sheet d with 
water. This is not by any means absolutely necessary, but causes 
a quicker gripping of bell-jar and rubber. The tap being in position 
No. 2, and connected with a water-pump in full action, filtering is 
commenced by first filling up the filter nearly full with the liquid 
to be filtered, and then establishing communication between the 
pump and the bell-jar by turning the tap to position No. 1. During 
the operation of transferring the precipitate to the filter, it is often 
necessary to lessen the rate of filtration by diminishing or destroying 
the difference of pressure outside and inside of the bell-jar. Instead 
of slipping off the rubber tubing connecting with the pump, this 
can be far more quickly and easily done by giving the tap a half- 
turn back to position No. 2, and thus allowing air to rush in both 
to the pump and to the bell-jar. The use of the knee-piece h will 
be now apparent, for by it the inrush of air is diverted away from 
the vessel inside, which might otherwise be blown against the side 
of the bell-jar, and upset or broken. As it is, however, the vessel 
inside is not at all affected by the inrush of air, however suddenly 
the tap be opened. In this connection another use of the knee- 
piece may be pointed out. Several of the otherwise very convenient 
and inexpensive high pressure water-pumps, now so much used, 
have a tendency to allow the water to run back under certain con- 
ditions, especially when a high degree of exhaustion is attained. 
Filtering directly into an exhausted flask, such an accident would 
cause the loss of an analysis, and a valve is therefore usually placed 
between the pump and the flask. Such a valve is quite unnecessary 
while using the above apparatus. The running back of the water 
can be at once stopped without diminishing the pressure inside the 
bell-jar by giving the tap a quarter turn, so as to admit air to the 
pump only. Even if water does run back into the bell-jar no harm 
