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Mr. Donovan's Filtering Apparatus. 



MR. DONOVAN S APPARATUS FOR FILTERING OUT OF CONTACT 

 WITH THE ATMOSPHERE. 



Where alkaline solutions in their caustic state are to be fil- 

 tered, and where it is required that the solution, after filtra- 

 tion, should still be in the caustic state, it becomes a matter of 

 some difficulty to exclude the atmosphere in such a way as to 

 prevent the absorption of carbonic acid from it by the alkali. 



Thus in the process for preparing pure potash, when the 

 carbonate of potash, the hydrate of lime, and the water, have 

 been exposed to each other's action, during a sufficient period, 

 it is found difficult to separate the lime by the ordinary pro- 

 cess of filtration. The mass is of so absorbent and pasty a na- 

 ture, that the liquor passes through the filter very slowly and 

 with great difficulty ; hence it is generally carbonated in its 

 passage, and the contrivances commonly resorted to, of cover- 

 ing the mouth of the vessel, &c. succeed very imperfectly. 



Difficulties of this kind compelled me to contrive some 

 means of conducting filtrations out of contact with the atmo- 

 sphere ; and I found that the end can be attained by a very 

 simple apparatus. 



The instrument consists of two glass ves- 

 sels, the upper one A has a neck at b, which 

 contains a tight cork so perforated as to ad- 

 mit one end of the glass tube c. The other 

 end of the vessel A terminates in a funnel 

 pipe, which fits into one of the necks of the 

 under vessel D, by grinding, or luting, or 

 by a tight cork. The vessel D has also 

 another neck c, which receives the other end 

 of the tube c, the juncture being secured by 

 a perforated cork, or by luting. The throat 

 of the funnel pipe is obstructed by a bit of 

 coarse linen loosely rolled up, and not 

 pressed down into the pipe. The alkaline 

 solution, containing the lime, is then to be 

 poured in through the mouth at b, the cork 

 and tube having been removed; and the 

 first drojipings are to be allowed to run to 

 waste, and are not to be received into the 

 imder vessel D. The parts of the apparatus 

 are now to be joined together, and the filtration may proceed 

 at the slowest rate without the possibility of any absorption of 

 carbonic acid by the alkali. 



It is scarcely necessary to explain the manner in which 

 this simple apparatus acts. It is obvious that no fluid can 



drop 



