116 THE AMERICAN MONTHLY [May, 



quired for the preparation of this medium. The medium can always 

 be made of the same consistency, as all of tlie agar that is added is dis- 

 solved. It remains free from precipitates when sterilized, and its nutri- 

 tive qualities are as favorable to bacterial growth as when it is prepared 

 after the original method. 



(i) The -preparation of the beef-infusioti-peptoiie. — The method 

 of preparing this liquid is practically the same as that already in use in 

 most laboratories. Finely chopped or ground beef (freed from fat) 

 is macerated in distilled water for from 12 to 18 hours in a cool place. 

 The distilled water is added in the proportion of 200 c. c. to each 100 grams 

 of beef. On the following day the liquid is separated from the meat 

 by straining it through a coarse linen. The simple beef-infusion thus 

 obtained should be equal in quantity to the amount of water added ; if 

 it is not the deficiency can be restored by the addition of distilled water. 

 To the beef infusion is added i per cent, peptone, \ per cent, sodium 

 chloride, and if it is desirable to make it alkaline a sufficient quantity of a 

 normal solution of sodium carbonate to give it a weak alkaline reaction. 

 The liquid is then boiled for thirty minutes in a water-bath, cooled, fil- 

 tered, and distributed in Erlenme3'er flasks plugged with cotton-wool. If 

 only a small quantity of agar is to be made at once, 250 c.c. is found to 

 be a very convenient quantit}' to put in each flask. It is sterilized by 

 boiling for one hour each day for three consecutive days. It need not 

 be sterilized if it is desirable to prepare the agar at once. As the beef- 

 infusion-peptone is also employed as a liquid medium in the cultivation 

 of bacteria, very little time is lost in preparing an extra quantity of this 

 liquid to be used in making the agar. 



(2) The preparation of the agar. — To an Erlenmeyer flask (a 

 glass-beaker or agate iron vessel may be used) containing beef-infusion 

 peptone, as prepared above, i per cent, of verv finely-chopped agar is 

 added. The flask is then placed in a water-bath and boiled vigorously 

 for two hours. At the end of that time the agar is all dissolved and 

 the liquid is allowed to cool. When a temperature of 40-45° C. is 

 reached, the white of egg is added in the proportion of one egg to 250 

 c.c. of the liquid. After the albumen is thoroughly mixed with the 

 liquid agar it is returned to the water-bath and again boiled for two 

 hours. It is of much importance that the albumen is evenly distributed 

 throughout the mass before it is coagulated. It is now read}' to be 

 filtered. The egg albumen is coagulated in very firm masses, leaving 

 the liquid perfectly clear. The coagulum is removed by filtering the 

 liquid through fine Japanese filter paper or a layer of absorbent cotton, 

 as a I per cent, solution of the agar does not pass readily through ordi- 

 nary filter paper. Should a weaker solution of the agar (^ to J per 

 cent.) be desired, its filtration can be accomplished by the ordinary 

 method. A hot filtering apparatus is not necessary. The clear filtrate 

 is now ready for distribution in sterile, cotton-plugged tubes. 



The agar is sterilized by discontinuous boiling in a closed water-bath 

 for tliree consecutive days. If small tubes have been used containing 

 not uKMc than seven cubic centimeters each, five minutes boiling each 

 day is sufficient. If larger tubes. are used they should be boiled for a 

 longer time. Or it may be sterilized by steaming each day for from 

 five to ten minutes after the agar has become liquefied for the same 

 number of days. After its sterility has been tested by allowing it to 



