82G SUMMARY OF CURRENT RESEARCHES RELATING TO 



pcpton, and 100 gm. of gelatin (or 10 gm. of agar) arc arldccl. This is 

 then placed in a water-bath (to which a large handful of rock salt has 

 been added, if agar is to be prepared) and the gelatin (or agar) melted 

 as rapidly as possible. The fluid is tlien neutralized by the careful 

 addition of sodium bicarbonate in solution, and the boiling continued 

 for a few minutes after, in order to precipitate the phosphates. 



The fluid is now cooled by running water, to such a temperature as 

 will not coagulate the white of egg, yet not enough to solidify it, when 

 the whites of two eggs, thoroughly beaten up, are mixed with it, and the 

 whole boiled for half an hour. 



Filtration which has usually been efi'ected by means of filter paper, 

 can be much more rapidly performed by the use of ahsorhent cotton in 

 large quantity. The pores of the paper become clogged by the fine pre- 

 cipitates and by the cooling of the medium, and even with the use of the 

 " hot funnel " the filtration is sometimes very slow. Cotton, on the other 

 hand, presents in its meshes a much larger surface for the entanglement 

 of the fine precipitates, and when used in large quantity, allows the 

 gelatin (or agar) even when not very hot, to flow through it rapidly. 

 The preparation of the filter is as follows : — The absorbent cotton is un- 

 rolled and sterilized in bulk in the hot-air chamber, care being taken 

 not to char it. A 6-in. (15 ccm.) glass funnel is packed full with the 

 dry sterilized cotton, placed in in layers, in such a way as to keep it well 

 out of the neck, and having no folds nor ridges of cotton next the glass, 

 through which the precipitates might pass into the receiving flask. The 

 neutralized culture medium, after being boiled with the white of egg, as 

 above described, is strained through coarse flannel into a flask, and j)oured 

 slowly upon the centre of the filter until the cotton is thoi'oughly soaked, 

 and the fluid begins to run into the flask below. This moistening causes 

 the cotton to sink considerably, and packs it in the funnel, and when packed, 

 the fluid filters through it almost as rapidly as it is poured into the 

 funnel. The funnel is now filled and the fluid filtered as fast as it will 

 run through. The first filtration seldom produces a clear medium, but 

 through the same filter the fluid may be poured again and again, each 

 time becoming clearer, and the moderate cooling which necessarily 

 occurs, does not sensibly retard the rapidity of filtration. When filtra- 

 tion is completed, a considerable portion of the medium entangled in the 

 filter can bo saved, by pressing upon the cotton with a sterilized glass 

 rod, gently at first and near the sides, then in the centre and with con- 

 siderable force. The gelatin or agar pressed from the cotton is some- 

 times cloudy, for which reason it is well to catch it in a separate flask. 



It not infrequently happens that gelatin which filters clear pre- 

 cipitates phosphates on boiling ; and that agar, on cooling, forms a 

 flocculent precipitate. To insure against filling tubes wath such media, 

 it is safest always to fill one tube with the medium, and by first cooling, 

 then by boiling and again cooling, to test the permanence of the trans- 

 parency obtained. Should these precipitates form, it will be necessary 

 to boil the gelatin in the flask, and to refilter it through a small plug of 

 dry cotton placed in a funnel ; while agar should be allowed to com- 

 pletely solidify, when it is again melted and filtered through a small 

 plug of cotton. The media are now ready for tubing and sterilizing in 

 the usual way. 



The large quantity of absorbent cotton used and the considerable 

 amount of medium lost, by remaining entangled in the meshes of the 



