126 
DR. P. P. FRANK LARD ON THE QUANTITATIVE ESTIMATION OF 
Rate .—’3 '5 strokes per minute. 
Plug mixed with 10 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
Colonies, 103, 114 . . Average = 108'5 colonies. 
Plates similarly prepared from control- 
plug gave colonies 7, 7 . Average = 7 ,, 
48 litresof air yielded/. 101'5 X 10 = 1015 ,, 
10 „ ” „ 211 
The experiments recorded above, although yielding in many cases very concordant 
results, point to several imperfections in the process, of which the more important are 
the following :— 
1. A very considerable volume of air has to be aspirated in order that the small 
fractions (yg- or - 2 J - 0 -), which can alone be examined by plate cultivation, shall yield a 
sufficient number of colonies for accurate estimation. Thus, even when 119 litres of 
air were employed, the plates (each of which represented -^g- of this volume) generally 
only contained from 20-30 colonies. 
2. A more serious objection is to be found in the difficuity of obtaining concordant 
plates put up from the water or broth with which the plug is mixed. This is due to 
the want of homogeneity caused by the material of the plug being suspended in the 
liquid. It was in order to reduce this disturbing suspended matter to a minimum 
that I constructed the plugs of sugar-powder and glass-wool coated with sugar, so 
that nearly the whole plug dissolved away, this construction being also intended to 
ensure the detachment of the organisms from the material of the plug by which they 
were arrested. But even the small proportion of insoluble matter in these sugared 
plugs is sufficient to prevent that uniformity in the plates prepared from one and the 
same plug which is essential to the quantitative accuracy of the process. 
That the discrepancies often observed in the case of the plates prepared with these 
glass-wool plugs is due to the suspended glass-wool and not to any inherent defect in 
the quantitative accuracy of the method of plate-cultivation in general is conclusively 
proved by the extraordinary concordance of the plate-cultivations of ordinary waters. 
Thus I may quote the following results from my examinations of the London waters 
for the Local Government Board :— 
