20G Report of the Departmes i of Bacteriology of the 



of egg, as generally recommended for ordinary bacteriological media- 

 Dextrose is added just before tubing. The reaction is adjusted 

 to 0.5 per ct. normal acid to phenolphthalein. The formula is given 

 in the fifth column of Table I (p. 203). 



VARIATIONS BETWEEN DIFFERENT BATCHES OF THE MEDIUM. 



A strong objection to this gelatin is its indefinite chemical com- 

 position. Variations in the composition might easily cause irregu- 

 larities in the counts. As a matter of fact, however, there has 

 seldom been any evidence of such variation; but in Table II is 

 given one instance where it was noticeable. This table shows the 

 counts obtained in a series of nine platings upon three batches of 

 soil-extract gelatin all made up from the same lot of soil-extract, 

 although from different packages of gelatin. Batch I was two and 

 a half months old at the time of use; batches II and III were made 

 up fresh, but batch II had been left in a warm room over night 

 before sterilization and a faintly noticeable decomposition had taken 

 place. It will be seen that the counts on batch II are considerably 

 lower than those on batch I. Batch III was used only six times. 

 Generally it gave a count intermediate between batches I and II. 



In this particular case the cause of the poor results from batch II 

 was undoubtedly its decomposition; but the decomposition was 

 so very slight that if it had not happened to be accompanied by gas 

 formation, it might have been overlooked. A similar accident 

 might easily occur in making up any batch without being noticed. 

 There are many other opportunities for such variation in composition 

 of the media. Agar is as liable to these variations as gelatin; again 

 and again some batch of agar under investigation, apparently made 

 up in exactly the same manner as the others, has proved unusually 

 satisfactory or else unusually unsatisfactory. These irregularities, 

 indeed, are great enough to make the result of any comparison 

 between two media unreliable unless more than one batch of each 

 medium has been used. 



This fact does not seem to have been fully realized by some 

 investigators. The six comparative tests given by Brown 22 were 

 presumably made with a single batch of each medium investigated, 

 although the author makes no statement to that effect. A glance 

 at his figures makes it plain that the variations between the counts 

 he obtained upon the different media are less than those shown in 

 Table II as occurring between two different batches of gelatin, 

 and likewise less than those known to occur between different lots 

 of agar media. 



22 See footnote 12. 



