New York Agricultural Experiment Station. 217 



showing that variations in the amount of asparaginate have no 

 appreciable effect upon the number of colonies that develop. For 

 quantitative purposes, 0.05 per ct. dextrose seems to be as good as 

 0.1 per ct. Possibly the dextrose could be entirely omitted without 

 causing a lower count. In fact a single test in which a formula 

 was used differing from that of Table I only in the absence of dex- 

 trose, resulted in exactly the same count as obtained with the use 

 of 0.1 per ct. dextrose.' 24 No further tests were made with this formula 

 as the colonies developing on it were all very small and alike in 

 appearance. 



A further series of tests was made to see if the formula of this 

 medium could be simplified. In Table VII the counts obtained by 

 the use of three agar media of more simple composition are compared 

 with those made on the ordinary formula. One of the simpler 

 media is a mixture of agar, tap-water, 0.1 per ct. of sodium asparagi- 

 nate and 0. 1 per ct. of dextrose ; the second the same with the dextrose 

 omitted; and the third a mixture of agar and tap-water alone. This 

 comparison was made in the hope that tap-water might supply all 

 the necessary mineral salts. Only two tests were made of tap- 

 water and agar alone, because, although this medium allowed a fairly 

 high count, the colonies were all small and of the same appearance. 

 When sodium asparaginate was added to this tap-water agar, how- 

 ever, the results were fairly satisfactory, and with the further addition 

 of dextrose more satisfactory still, but the colonies were not even 

 then as large as when the formula given in Table I was used, and 

 the count was usually lower. This attempt at simplification cannot 

 be considered a success. 



It has already been mentioned that a most important point in 

 the composition of the asparaginate agar is its reaction. There is 

 very little data available to prove this point by direct comparison, 

 although it has been well established in the course of the present 

 work. The fact was learned largely by noticing that whenever the 

 medium was made up by accident with a reaction as high as 1.5 

 per ct. normal acid, the counts obtained were always much lower 

 than expected. This was so very evident that a few additional 

 direct tests were considered enough to settle the matter. They are 

 given in Table VIII. The four tests all agree in showing that the 

 count is about twice as high when the reaction is 0.8 per ct. as when 

 it is 1.5 per ct. Additional weight is given to these figures by the 

 fact that tests Nos. 1 and 2 were made with different batches of media 

 from those used in Nos. 3 and 4, the media used in the last two tests 

 having a slightly different formula from usual (0.05 per ct. dextrose 

 and 0.2 per ct. asparaginate). No media were tested out with a 

 reaction more alkaline than 0.8 per ct. acid, because of the danger of 

 losing the ammonia of ammonium phosphate unless the medium was 



s * This test is not included in any of the tables. 



