362 MICRO-ORGANISMS IN WATER 



be as small and closely packed as possible. If the 

 individual centres are able to develop into large colo- 

 nies, they will naturally extend beyond the margin of 

 the screened part, the sharpness and definition of the 

 letters being thereby impaired. Even ten minutes' 

 direct insolation is sufficient to exhibit the letters of 

 the screen, although the production of colonies is not 

 altogether inhibited by this short exposure, but their 

 development is so retarded that even after twenty-four 

 hours' incubation the protected part of the plate is well 

 defined by the far more luxuriant growth of the colo- 

 nies there. It is obvious that the process may also be 

 reversed by pasting opaque letters on to the bottom 

 of the transparent dish ; and in this case the letters will 

 appear as colonies on the film, instead of as sterile areas 

 as in fig. 22. 



In order to dispose of the suggestion that the local 

 development of the colonies in these dish-cultures 

 might be due to the difference in temperature between 

 the protected and exposed portions of the agar-agar, 

 similar dishes were insolated whilst lying in a vessel 

 of water ^ metre in depth, but the results showed no 

 deviation from those previously recorded. This points 

 (says Buchner) not only to temperature having nothing 

 to do with the effect of insolation, but also to the fact 

 that the bactericidal agency of the latter is not inter- 

 fered with in passing through such a depth of water. 

 (In this connection, however, see also Arloing, p. 342, 

 Buchner, p. 372, and Procacci, p. 374.) 



We must now return to the anthrax bacillus, which 

 was the micro-organism selected by Moment for his 

 carefully conducted investigation. 1 The majority of 



1 ' Action de la Dessiccation de 1' Air et de la Lumiere stir la Bacteridie 

 charbonneuse filainenteuse,' Annales de Vlnstitut Pasteur, vol. vi., 1892, 

 p. 21. 



