Report on the Bacteriology of Water. 319 



exposed in contact with air nearly all the spores are killed in the 

 time given. As will be shown later, the thickness of the glass of 

 which the tubes were composed is no doubt an important factor, and 

 probably all the spores in the exposed unsealed tube would have been 

 killed had the glass been thinner, as they certainly could have been 

 by a longer exposure.* 



II. 



The following series of experiments were carried out during Feb- 

 ruary and March of this year (1893) to obtain some information as 

 to the time of exposure necessary to kill the spores of Bacillus 

 anthracis, for I found it desirable to make myself as well acquainted 

 as possible with the power of the solar rays in this respect, in order 

 to utilise the experience in succeeding work. As the table shows, I 

 also tried comparisons between the action of direct and that of 

 reflected sunlight. As the experiments proceeded, it turned out that 

 several difficulties have to be met in attempting to compare the action 

 on two or more different plates exposed side by side. 



It seems impossible to ensure absolute similarity between any two 

 plates, for the following reasons : 



1. The difficulty of distributing the spores in equal quantities, and 

 at equal distances apart in the agar. The best results were obtained 

 by pouring the agar on all the plates from one large tube, in which 

 the infected melted agar is thoroughly shaken ; but even then it was 

 impossible to be sure that the agar film in each plate was of equal 

 thickness. Of course practice and experience enable one to pour 

 approximately the same quantity of the agar into each plate, but this 

 does not entirely overcome the difficulty. 



2. Even very careful selection from a large number of the Petri's 

 dishes does not secure that each dish used shall have a perfectly 

 plane glass face (to be exposed), of equal thickness, and identical in 

 its properties towards the light. Here, again, therefore, one had to 

 be satisfied with as close approximations as possible. 



It was owing to these difficulties that I hit upon the device of 

 employing one plate with several square or circular " windows " cut in 

 its covering, at equal distances apart. After exposing all the 

 "windows" for, say, half an hour, one was then covered; after a 

 further exposure of half an hour, a second one was covered, and so 

 forth (or, conversely, the windows uncovered in succession), as in 

 the cases marked la to Id, and 5a to 5d, &c., on Table B. 



But another difficulty now made itself evident, namely, that as the 

 intensity of the solar light may vary considerably from time to time 

 not only owing to altitude, but also to differences in the atmosphere, 



* It may be pointed out that these results confirm those of Eoux, ' Ann. Past. 

 Inst.,' 1887. 



