296 BACTERIOLOGY. 



each tube is smaller than is usual ; this is in order to 

 allow for the subsequent addition of the urine and 

 serum. 



" 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 far as is possible any particles in suspension, 

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

 The mixture of urine 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 pre- 

 vious sterilization by steam of the cylinder and receiv- 

 ing-flask, besides others which will occur to any bacteri- 

 ologist. 



" 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 eifected 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 uniform mixture, and is 

 then placed in an inclined position to allow the agar to 



