262 BAOTEBIOLOGY. 



"The blood-serum, which need not be free from cor- 

 puscles, is first passed through white sand, which is 

 supported in a funnel by filter-paper, in order to re- 

 move as much as possible any particles in suspension, 

 and is then mixed with half its volume of fresh urine. 

 The mixture of uriue and blood-serum is next filtered 

 by suction through an unglazed porcelain cylinder into 

 a receiving flask, such as chemists use for similar pur- 

 poses by means of a water vacuum pump. This frees 

 the mixture from bacteria. 



"The usual precautions are, of course, taken to pre- 

 vent the contamination of the filtrate, such as the previous 

 sterilization by steam of the cylinder and receiving flask, 

 besides others which will occur to any bacteriologist. 



"To the agar in each test-tube, which is fluid and of 

 a temperature of about 40° C, there is added about 

 one-third to one-half its volume of the filtered mixture 

 of urine and blood-serum. This is conveniently accom- 

 plished by pouring the mixture from the receiving flask 

 through the lateral tube, inserted near its neck directly 

 into the tubes. The preliminary melting of the agar 

 is best effected in the steam sterilizer in order that any 

 organisms which have found lodgement in the cotton 

 plugs of the tubes may be destroyed. When the agar 

 is melted it is cooled and kept fluid by placing the 

 tubes in a water-bath at 40° C. Each tube, after the 

 addition of the urine and serum to the fluid agar, is 

 quickly shaken to insure a good mixture, and is then 

 placed in an inclined position to allow the agar to 

 solidify with a slanting surface. Wlien the medium 

 in the tubes has solidified the tubes are placed in the 

 incubator for about twenty-four hours to test for con- 

 taminations, after which they are ready for use." 



