340 Profs, Percy Frankland and Marshall Ward. 



IX. 



On August 14bh, three flasks were prepared and exposed as before ; 

 A' with mirror beneath, B' with no mirror, and C' wrapped up. 

 Fairly bright sunshine prevailed during the exposure from 11.15 to 

 4.30 an occasional cloud obscuring the sun. 



Two samples showed that A' started with 1755 per c.c. and 1404, 

 the mean being 1578 per c.c. After five hours' exposure, over the 

 mirror, plates were again taken. Three plates yielded 1326, 780, and 

 858 per c.c., the mean being 988 per c.c., which looks as if a perceptible 

 reduction had occurred. 



Two plates from B' at the start gave 680 and 2176, the mean being 

 1423 per c.c. After its five hoars' exposure, without a mirror, three 

 samples yielded 918,476, and 476 per c.c., the mean being 623 per c.c., 

 and again suggesting effect of bactericidal rays. 



Plates from C' at the beginning gave 1156 as the number to start 

 with, and after the five hours side by side with the other flasks, but 

 protecbed from the sunshine by foil and paper, samples gave 3240, 

 2052, and one uncountable. The mean, = 2646 per c.c., suggesting a 

 perceptible increase. 



Here, again, it was evident that liquefaction took place much more 

 rapidly on the plates from the unexposed flask than on those from the 

 flasks exposed to light. 



The chief difficulty with these mixed plates is always that caused 

 by the liquefying forms, one of which was especially troublesome, 

 often coming on so rapidly that a plate which looked " safe " at a 

 given time would be mined three or four hours later. 



The foregoing results are summarised in the following Table I. 



Without attempting to lay too much stress on the actual numbers 

 in this series, it is pretty evident that if we take the totals or the 

 means of the numbers of bacteria obtained from the water by taking 

 three samples from each flask at each period of examination, we get 

 at least some information as to the rate of action of the light on the 

 total organisms. 



Put thus, the facts run as follows : Of the nine samples taken at 

 the start, four were not counted, as they liquefied too rapidly. The 

 average of the other five gave 1434 per c.c. 



Flask A', after five hours' exposure over a mirror, gave 988 per c.c. 

 as the mean of three samples. 



Flask B', after five hours' exposure without a mirror, gave 623 per 

 c.c. as the mean of three plates. 



Flask C', not exposed, but otherwise treated similarly, gave 2646 

 as the mean of two plates. 



It seems impossible to doubt, therefore, that the exposure to light 

 reduced the numbers by nearly one-half. But this proportion becomes 



