216 BACTERIOLOGY. 



bouillon 1 or 2 per cent., as desired, of thread agar, 

 melting it by placing over a free flame or in the auto- 

 clave or steam sterilizer. When the agar is brought 

 into solution over a free flame there may be consider- 

 able loss of fluid by evaporation. This should be com- 

 pensated for by adding additional water before boiling. 

 Agar may be added directly to the meat infusion along 

 with the peptone and salt. Indeed, this is an advan- 

 tage, as agar-agar is very difficult to bring into solution, 

 and is not injured in the least by prolonged boiling. 

 Glycerin agar is simply nutrient agar plus 3 to 6 per 

 cent, of glycerin. It is added to the hot nutrient agar 

 just previous to putting it in the flasks. Nutrient agar 

 begins to thicken at a fairly high temperature, and 

 should be filtered as hot as possible. When small 

 amounts are made it is well to place the filter and re- 

 ceiving-flask in the sterilizer while filtering. 



Milk. This fluid is a good culture medium for most 

 pathogenic bacteria. It should be obtained as fresh as 

 possible, so that but little bacterial change has occurred. 

 It is first put in a steam sterilizer for fifteen minutes 

 and then put in the ice-chest for twelve hours, to allow 

 the cream to rise. The milk is then siphoned off from 

 below the cream into a flask and its reaction tested. A f ter 

 correction it is put in tubes or flasks and sterilized. 



Potatoes. Potatoes are used for some special pur- 

 poses. The potatoes may after thorough scrubbing and 

 removal of "eyes" be soaked in bichloride of mer- 

 cury (1 : 1000) for twenty minutes, and then sterilized 

 on three consecutive days for one-half hour in the 

 steam sterilizer. To use they are cut in thick slices and 

 put in deep Petri dishes. For more careful work the 

 potatoes are first cut into proper sizes for tubes or 



