84 TEXT-BOOK OF BACTERIOLOGY. 



with the gas-pipe and feeds about thirty little flames. These make 

 the water boil, and the inner glass funnel is thus surrounded by 

 boiling- water. This, of course, very considerably accelerates the fil- 

 tration of the gelatin, and if one takes care always to have a suffi- 

 cient quantity of water between the two walls of the funnel, the 

 apparatus gives no occasion for complaint. Yet the filtration can 

 easily be performed without its aid if the unfiltered gelatin in the 

 flask be kept warm in the water bath. In Koch's laboratory the 

 hot-water funnel has hardly ever been used for years past. 



The filtered gelatin must be perfectly clear, transparent as 

 water, and but slightly tinged with yellow; it should contain noth- 

 ing flaky, should not become turbid when boiled or cooled, and 

 should show a clearly alkaline reaction. 



One may test the two last-named qualities by letting the first 

 liquid that passes through the filter run into a test-tube, testing 

 its reaction, and heating it until it boils. If the gelatin proves acid 

 the filtration must be interrupted and the requisite quantity of 

 carbonate of soda solution added, after which it should be boiled 

 again for about a quarter of an hour. This acidity is sometimes 

 found when the solution was decidedly alkaline after its first neu- 

 tralization. The chief cause for this change in reaction is that dur- 

 ing the boiling, meat acids and acid salts which were not present 

 before have been set free. 



It is more difficult to get rid of any turbidity that may appear 

 in the gelatin, since the causes are various. 



Perhaps the filter was not fine enough, may have been torn at 

 the point in twisting it at first, or it may have burst when the gel- 

 atin was poured into it. Carefully examine the filter before using 

 it, and only admit the liquid slowly and cautiously. It is desirable 

 to increase the strength of the filter by putting a small protecting 

 cap of cotton-wool at the bottom of it. 



It sometimes also happens that the gelatin is turbid only when 

 it first begins to run through, while that which follows is clear; 

 that is, when the filtering paper is poor and its pores have to un- 

 dergo an alteration before they can hold back the little coagula. 

 Or it may be that the gelatin is too strongly alkaline. This is fre- 

 quently the result of its being too rich in carbonates. 



If the mass has to be treated anew, the carbonic acid is driven 

 off, the combinations are dissolved, the solution is made turbid. In 

 all such cases the evil may easily be removed: readjust the reac- 

 tion, employ good filters, and recommence the filtration. 



Yet it sometimes happens that none of these means prove effi- 

 cient. Filter as often and as carefully as we may, the gelatin re- 



