568 S. HENRY AYERS AND COURTLAND S. MUDGE 



MILK-POWDER AGAR 



Medium A . Ingredients for one liter of medium 



{5 grams skimmed-milk powder 1 In 25Q cc dig . 



(a) <j 1 gram sodium dibasic phosphate. (Sorensen's V ... , 



[ phosphate 1 ) (Na^HPC^ + 2H 2 0) J water 



|5 grams peptone K m ^ digtilled water 

 [3 grams extract J 



Mix (a) and (b) and add 500 cc. of double strength (3 per cent) 

 washed-agar solution. 



DETAILED DIRECTIONS FOR PREPARATION OF 1 LITER OF MEDIUM 



Milk-powder solution 



The detailed directions must be followed accurately if satisfactory 

 and constant results are to be obtained. The medium is very easy 

 to prepare when the various steps are understood and the process 

 completed once. It may appear complicated because of the 

 complete details which are given of each step in the process and 

 which make the preparation of the medium appear somewhat 

 long. 



1 Sorensen's phosphate is sodium dibasic phosphate with 12 molecules of 

 water of crystallization (Na 2 HP0 4 + 12H 2 0) which has been air dried so that 

 instead of containing 12 molecules of water it has 2 molecules. Anhydrous so- 

 dium dibasic phosphate takes up water easily as it stands in the laboratory and 

 that containing 12H ; loses water easily, so that neither of these can be de- 

 pended upon to be definite in regard to water content. To overcome this diffi- 

 culty, Sorensen air-dried Na 2 HP0 4 + 12H 2 for about two weeks and found 

 that the water of crystallization was reduced to 2H 2 and that it remained 

 practically constant. Sorensen's phosphate can be purchased or for practical 

 purposes prepared in the laboratory by taking the ordinary dibasic phosphate 

 with 12H 2 and spreading it out on filter paper and allowing it to remain at 

 room temperature in a dry place for fourteen days. This phosphate (Na 2 HP0 4 + 

 2H 2 0) is used in the preparation of the medium so that a definite amount can 

 be weighed out at any time and the milk-powder solution will always have prac- 

 tically the same hydrogen-ion concentration. If this phosphate is purchased, 

 care should be taken to be sure it contains two molecules of water and is not 

 anhydrous. 



