1900] MICROSCOPICAL JOURNAL. IT 



Usually, on account of the liability to secondary pr-a 

 cipitates, and because the agar is never so transparent 

 when filtered immediately as it is when the filtration is 

 deferred until after the first sterilization, I do not filter 

 at once, but merely strain out the coarser flocculi by run- 

 ning the medium through losely packed cotton, sterilize 

 in flasks, allow the flasks to stand in the sterilizer and 

 slowly cool, and wait until the following day before fil- 

 tering through paper. Filtration is then still more rapid, 

 if care is taken to bring the temperature of the mass up 

 to the boiling point in the sterilizer before commencing 

 the filtration, and the product is always transparent. 



The coarser precipitates which occur on sterilization are 

 usually due to the coagulation of albumin which has es- 

 caped coagulation at the time of the preparation of the 

 medium ; but the troublesome ones are of more doubtful 

 origin ; probably they consist, in the first place, of very 

 fine flocculi which pass through the filter on the first fil- 

 tration, aud, in the second place, of salts which are held 

 in solution during the first filtration but which as a result 

 of changes in the reaction, oxidation, or because of less- 

 ened solubility in the cold medium and their presence to 

 supersaturation, are deposited as the medium cools. But 

 whatever their nature and cause I have beenunableto avoid 

 their appearance altogether save by the method just de- 

 tailed. When present in only small amount and sterili- 

 zation is not too much prolonged, (ten minutes) if the 

 tubes are quickly cooled they cause no preceptible sedi- 

 ment and only a slight opalescence in the fiuished product 

 and are then really not objectionable, though I always 

 prefer to have my media perfectly transparent if possible. 



Eggs are not needed to clear the agar when made by 

 the above process, the albumin in the meat juice being 

 sufficient for the purpose. 



If it be desirable to make agar from boullion it is only 

 necessary to rub up the powdered agar with a little of 



