1894] on the Action of Light on Bacteria and Fungi. 265 



the question whether the light-action is really direct on the spores, 

 or due to some poisoning action owing to products of oxidation of 

 the food-materials. 



This problem I attacked in the following manner. Spores were 

 shaken up in pure water, and the mixture poured into sterile shallow 

 flat glass dishes — the Petri dishes used for plate-cultures — and dried 

 there in a hot oven at 60° C. to 70° C, or even higher ; this does not 

 injure the spores, but renders them air-dry, and they stick as a thin 

 powdery film to the bottom of the plate in this condition. 



I then prepared several plates of agar films, sterile and without 

 spores, and proceeded further according to the following argument : — 

 If the solar action depends on the formation of some poisonous 

 oxidation product in the agar, then, if I expose one of these sheets of 

 agar, covered with a stencilled letter, to the sun, and then superpose 

 it on an un-exposed film of the spores only, the latter ought to refuse 

 to germinate on the letter-shaped area of the agar which had been 

 exposed, whereas they should grow on the parts of the agar which 

 had been protected from the sun. 



But on trying the experiment, such proved not to be the case. 

 On the contrary, the spores germinated equally well all over the agar 

 film, on the parts exposed as well as on those not exposed. In other 

 words, the agar is not rendered in any way a worse pabulum for the 

 spores by exposure to the sun's rays — a fact quite in accordance with 

 numerous other experiments I tried. 



This result by itself, however, is far less conclusive than when 

 taken with the reciprocal experiment, as follows : — 



Side by side with the sterile agar film above referred to, I also 

 exposed a film of the dried spores — without agar — for the same 

 period, and under the same conditions as to covering, stencil-letter, 

 &c. If, now, the solar action is direct on something in the spore 

 itself, then those spores in that letter-shaped area of the film to which 

 the stencil-plate allows the sun's rays to have access, ought to be 

 killed : consequently, on superposing a non-exposed film of agar on 

 to one of these exposed films of spores, I ought to obtain a trans- 

 parent letter after the spores around have grown out into opaque 

 colonies. 



And such proved to be the case, as the accompanying photograph 

 shows. [Experiments and photographs demonstrated.] 



These reciprocal experiments taken together, conclusively show 

 that the light-action is really direct on the spores themselves, and 

 that the solar rays do not perceptibly affect the food- value of the agar 

 films. 



The same result is also obtained by taking some of the germinating 

 spores from the non-exposed parts of a gelatine or agar plate in the 

 previous experiment, and placing them on the exposed area where all 

 the spores have been killed : they grow and flourish on this exposed 

 area as well as they do on the non-exposed one, whence may be 

 inferred that the bactericidal action, whatever it is, is not a mere 

 Vol. XIV. (No. 88.) t 



