1893.] 



Action of Light on Bacillus anthracis. 



31 



fere with the results obtainable when the spores are distributed 

 through a matrix of agar. Finally, it was possible that the aerobian 

 dry spores would not germinate with a layer of agar poured over them, 

 although I employed the thinnest coverings possible. 



On the other hand, positive results with the dried spores would be 

 conclusive as to the main point at issue ; the actual results seem to 

 me to set the matter at rest, for they prove that the action of the 

 solar rays is direct on the ripe spores, and that the food material of the 

 culture media is not appreciably concerned at all (see Table III). 



Passing over a number of experiments where gelatine was em- 

 ployed, and which were rejected because this medium is too soft and 

 runs too easily to yield good prints, I select the following: — 



On January 20 two plates of plain agar were made as thin as 

 possible ; one (Table III, A) had a stencil C, the other (Table III, B) 

 an H, for its letter. On January 22 two plates were also made as 

 follows: — Two Petri's dishes were heated to 130° C. in the hot-air 

 oven : when the temperature of the oven had fallen to 90° C, each 

 plate received a few drops of sterile distilled water strongly infected 

 with anthrax spores, and the fluid was distributed as evenly as possible 

 over the base of the hot dishes. The whole was then left to cool, 

 when each dish had a thin film of the dry spores sticking to it. One 

 (Table III, C) was covered with a stencil letter H, the other 

 (Table III, D) with a small B. 



All four plates were exposed daily till January 27, but the weather 

 was so dull that they received practically no sun on the 20th to the 

 26th inclusive : the 27th was very bright, however, and each plate 

 had at least five hours' good insolation that day. At sundown I 

 distributed spores in water on the two pure gelatine plates, and a 

 thin layer of gelatine over the two plates of pure spores. 



On the 29th the letter H was clearly visible on the one plate of the 

 dried spores (Table III, C), and the B faintly marked and blurred on 

 the other (Table III, D), proving that spores had been killed over 

 the area — H and B — exposed to light, but not elsewhere ; whereas 

 the two plates made with agar only were uniformly covered with the 

 anthrax growth, showing that the spores had germinated equally all 

 over, and therefore that the agar exposed to the sun (on the areas C 

 and H) was not rendered incapable of supporting their growth. 



On January 27 I made two thin plates of agar only, without 

 spores, and two plates of spores only, dried at 70° C, as described 

 above. 



Two of these plates (Table III, E and G) were exposed forthwith 

 at 1 p.m., and received over two hours' direct sunlight. Each was 

 then — about 3.30 p.m. — put in the incubator, after receiving a charge 

 of gelatine (on the spores) and of spores in water (on the agar) 

 respectively. 



