416 G. s. "mLSON 



made, but, while granting that this may be true, it would appear 

 more probable that the difference is due chiefly to a lack of uni- 

 form mixing of the emulsion with the agar in the plate prepara- 

 tions. An examination of these showed that the colonies tended 

 to be aggregated in groups, while in the roll tubes they were dis- 

 tributed equally throughout the agar. The effect of this crowd- 

 ing is, as is afterguards demonstrated, to lead to the failure of 

 certain bacilU to develop into single colonies; hence a smaller 

 number of colonies will result than would otherwise be the case 

 when the bacilli are separated by a sufficient space from each 

 other. If, in the plate preparations, uniform mixing could be 

 insured, this difference would probably disappear; incidentally, 

 however, the difficulty of insuring such a mixture is by no means 

 imaginary, and whatever method be employed for gaining this 

 purpose an unnecessarily large amount of time must be spent 

 over each plate. 



To sum up briefly, then, it is claimed that roll tubes possess 

 certain advantages over plates, the chief of which is undoubtedly 

 that the count tends to be higher — probably at least 5 per cent — 

 and therefore presumably more accurate. The only objection 

 to their use is that they take a slightly longer time to put up ; this 

 is more than counterbalanced, however, by the greater rapidity 

 and ease with which they can be counted. 



The nature of the diluent 



For counting emulsions dilution has been preferred to direct 

 sowing. For this purpose a suitable number of drops is delivered 

 into a flask containing the diluent, and if necessary a further 

 dilution is made by the use of a second flask. In every case a 

 fresh pipette is used for each dilution. 



In the early stages of this work attention was directed to the 

 nature of the diluent used in preparing the emulsions for counting. 

 A review of the literature shows that the most popular fluids 

 employed for this purpose have been saline, distilled water, and 

 tap water. A preliminary experiment undertaken to ascertain 

 whether any material difference could be discerned between these 



