350 APPLICATION OF METHODS OF BACTERIOLOGY 



serving the purpose), and nutrient agar-agar. The urine and 

 blood-serum are collected without special aseptic precau- 

 tions, and subsequently freed from bacteria by filtration 

 through unglazed porcelain. Frequently this is the tedious 

 part of the process, as the serum and urine pass very slowly 

 through the porcelain filters generally employed in labora- 

 tories. Wright recommends a filtering cylinder manufac- 

 tured by the Boston Filter Company as an apparatus that 

 not only gives a sterile filtrate, but also permits of very 

 rapid passage of the fluid. 



The details of the method as given by Wright are as 

 follows: "A liter of nutrient agar is prepared in the usual 

 manner, and after filtration it is evaporated to about 600 c.c. 

 This concentration is desirable, so that after dilution with 

 the urine and serum the medium may be sufficiently firm. 

 This concentrated agar is then run into test-tubes and the 

 whole sterilized by steam. The quantity of agar placed in 

 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 corpuscles, 

 is first passed through white sand, which is supported in 

 a funnel by filter-paper, in order to remove 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 purposes, by means of a water- vacuum pump. 

 This frees the mixture from bacteria. 



"The usual precautions are, of course, taken to prevent 

 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. 



