586 S. HENRY AYERS AND COURTLAND S. MUDGE 



The range in the percentage of lactose in the medium would 

 be only 0.03 per cent and in protein also 0.03 per cent. These 

 differences are so slight that they would exert no influence on 

 the medium. It is realized that there may be other differences 

 in skimmed-milk powder, small differences which may not be 

 apparent from the ordinary chemical analysis, and which may 

 be ascribed to the condition of the milk before it was dried, or 

 perhaps to methods of drying. 



We have obtained the best results when using a skimmed-milk 

 powder made by the spray process, and suggest that if milk 

 powder is used in a standard medium a spray-process powder 

 be specified, and one prepared by some one company, perhaps 

 even from some special milk. 



THE ESTIMATED COST OF DIFFERENT AGAR MEDIA 



We are not prepared to say just how important the cost of 

 a medium is, but it will always be a factor and therefore must 

 be considered. 



The cost of different agar media has been calculated on a 

 basis of the cost of ingredients shown in the list. 



Cost of ingredients 



Peptone $4.80 for 500 grams 



Meat extract 3.70 for 1 pound 



Dry yeast 3.75 for 500 grams 



Xa 2 HP0 4 + 2H 2 1.50 per pound 



Skimmed-milk powder 50 per pound 



Lean beef 35 per pound 



Agar (shred) 90 per pound 



Approximate cost of agar media 



per liter 



Standard infusion agar about SO . 46^ 



Standard extract agar about 10 



Milk-powder agar "A" 10j 



Milk-powder agar "B" 06j 



Milk-powder yeast agar 11^ 



In making calculations we have not figured the cost of washing 

 agar. ; This should not add much to the cost. From these 

 figures it may be seen that the milk-powder media cost about 



