Report on the Bacteriology of Water. 



307 



the plate of gelatine. Not a trace of anthrax could be found in the 

 clear area, even with the microscope, while the grey and almost 

 opaque appearance of the rest of the plate was due to innumerable 

 colonies of that organism which had developed in the interval. 



It was impossible to incubate the plate longer for fear of liquefac- 

 tion, whence the sceptical may reply that the anthrax exposed to 

 light was only retarded ; the experiments with agar show that such 

 is not the case, however, and that if the insolation is complete the 

 spores are rendered incapable of germinating at all, as proved by 

 removing pieces of the clear agar or gelatine and attempting to 

 make tube cultures from them : in all cases where insolation is com- 

 plete they remain sterile. 



The chief value of these gelatine plate exposures in November, 

 however, is that they prove conclusively (1) that the rays of a winter 

 sun are capable, even if reflected, of killing the spores, and (2) that 

 it is really the solar rays which do this directly, and not any effect of 

 a higher temperature, since the gelatine remains solid throughout. 



Experience has shown, however, that some precautions are neces- 

 sary in selecting the anthrax cultures employed for these experiments 

 with gelatine. The light certainly retards or kills (according to its 

 intensity or the length of exposure) virulent spores, but if one takes 

 the spores, mixed with vegetative bacilli, direct from a thoroughly 

 liquefied gelatine culture, or from a bouillon culture, the plates are 

 apt to be liquefied too rapidly for the proper development of the light 

 print, evidently because so much of the liquefying enzyme is carried 

 in when inoculating the plates. The same danger is run when active 

 bacilli alone are employed. 



The best method of avoiding these disadvantages has been found 

 to be the following, and it has the additional merit of enabling us to 

 prove, beyond all doubt, that the ripe spores of Bacillus anthracis are 

 really'inhibited or killed by sunlight. 



A few c.c. of sterile distilled water in a tube are thoroughly 

 saturated with the anthrax spores taken from an old culture which 

 has never been exposed to light, and the tube placed for twenty-four 

 hours at 56° C. ; this kills all immature spores, bacilli, and enzymes, 

 and leaves us with a crop of the most resistant and fully matured 

 virulent spores. 



Experiments with such spores have been made to determine the 

 relative power of the different rays of the spectrum to destroy the 

 anthrax. 



It is necessary to note first, however, that in experimenting with 

 the electric light, although but few exposures have been made as yet, 

 it is evident that its effects are feebler than those of the winter sun. 



At present it has only been possible to observe that the inhibiting 

 effects are stronger at the blue end of the spectrum than at the red, 



