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NOTES AND ABSTRACTS IN CHEMISTRY AND PHARMACY. 
The filtration is made in the following manner:—The flask standing in the 
metallic or porcelain vessel is connected by means of the slightly drawn- 
out tube k with the caoutchouc tube h attached to the pump, the cock a having 
been previously opened and the properly fitted moistened filter filled with the 
liquid to be filtered. As usual, the clear supernatant fluid is first poured upon 
the filter ; in a moment or two the filtrate runs through in a continuous stream, 
often so rapidly that one must hasten to keep up the supply of liquid, since it 
is advisable to maintain the filter as full as possible. After the precipitate has 
been entirely transferred, the filtrate passes through drop by drop, and the ma¬ 
nometer not unfrequently now shows a pressure of an extra atmosphere. The 
filter may be filled (in fact, this is to be recommended) with the precipitate to 
within a millimetre of its edge, since the precipitate, in consequence of the high 
pressure to which it is subjected, becomes squeezed into a thin layer broken up 
by innumerable fissures. As soon as the liquid has passed through and the first 
traces of this breaking up become evident, the precipitate will be found to have 
been s > firmly pressed upon the paper, that on cautiously pouring water over it 
it remains completely undisturbed. The washing is effected by carefully pour¬ 
ing water down the side of the funnel to within a centimetre above the rim of the 
filter : the washing-flask for this purpose is not applicable ; the water must be 
poured from an open vessel. After the filter has in this manner been reple¬ 
nished four times with water and allowed to drain for a few minutes, it will be 
found to be already so far dried, in consequence of the high pressure to which 
it has been subjected, that without any further desiccation it may be withdrawn, 
together with the precipitate, from the funnel, and immediately ignited, with 
the precautions to be presently given, in the crucible. 
The saving of time which may be effected in analytical operations by the 
employment of this apparatus, is very considerable. Even with the most re¬ 
fractory precipitate, such as hydrated oxide of chromium, complete washing 
may be effected in one-tenth part of the time usually required. Moreover, it 
is possible, in cases where it is necessary to calcine the precipitate, to omit alto¬ 
gether the usual drying in a water-bath or oven. From one to five minutes’ 
draining in the funnel is quite sufficient. The portion of filter-paper free from 
precipitate is tightly wrapped round the remainder of the filter, and the whole 
cautiously carbonized in a platinum crucible, raising the heat towards the end. 
The filter-pump, moreover, is exceedingly serviceable in separating precipi¬ 
tates or crystals from syrupy mother-liquors. Thus honey-sugar may be so 
completely separated from the thick viscid liquid in which it forms, by a filter 
of coarse grey paper, that it remains only slightly coloured, and by a single 
crystallization from alcohol may be obtained in small white shining needles. 
And since the bulk of the moist precipitates, particularly that of the more 
gelatinous, is so much diminished under the high pressure, the precipitate only 
occupying one-third to one-sixth of its bulk under ordinary circumstances, a 
filter of one-third to one-sixth of the size usually employed may be taken, and 
thus the amount of ash proportionately lessened. 
A s the water air-pump suffers no injury from the presence of corrosive va¬ 
pours or gases, we can equally well employ it to filter liquids containing nitrous 
acid, sulphurous acid, fuming nitric acid, chlorine, bromine, volatile chlorides, 
etc. 
In such cases Bunsen employs a cylindrical glass vessel, drawn out to a narrow 
tube at the lower end, in place of a funnel. In this drawn-out portion, a plate, 
one-twentieth of an inch in thickness, of artificial pumice, such as is used by 
polishers, is packed water-tight by means of asbestos. Upon such a filter, by 
means of the pump, crystals of chromic acid may be drained, then washed with a 
small quantity of fuming nitric acid free from nitrous acid, and perfectly dried 
by warming the tube, while a current of dry air is drawn through. 
