4 The Influence of Light on Bacteria. 



the wrapping from the heat of the surface on which they 

 rest than others not so wrapped. The temperature attained 

 under these circumstances will depend, in fact, more on the 

 height of the column of fluid than on the mere difference of 

 wrapping or no wrapping. The high a priori method 

 which Dr. Downes adopts in his communication is, I venture 

 to think, not quite appropriate in an inquiry, in which 

 direct experiment is applicable, and can, indeed, alone be 

 conclusive. An illustration of the danger in applying this 

 method may be taken from the first paper of Messrs. Downes 

 and Blunt (Proc. Roy. Soc, 1877, pp. 499, 500). They found 

 that, of tubes containing urine exhausted with a Sprengel 

 pump, those which were insolated became turbid sooner than 

 those which were encased. This experiment may not have 

 proved that insolation favours the development of bacteria, 

 but it surely may be taken as showing that insolation per 

 se is not excessively destructive. 



I may have been wrong in attributing too much influence 

 to an elevated temperature per se ; but I must still insist 

 that Messrs. Downes and Blunt gave too little consideration 

 to it as at least a disturbing element, recognising it only as 

 a condition favourable to development. 



In my previous paper I did not venture to deny to direct 

 sunlight any influence whatever inimical to the develop- 

 ment of bacteria, though I did not think that that inimical 

 influence was established by the experiments described. I 

 have felt it incumbent on me to repeat, with variations, the 

 investigations previously reported, and though perhaps even 

 less disposed than I was then to consider light a mere 

 neutral factor, I am still compelled to repeat that bright 

 light, and even direct insolation, need not prevent the 

 development of bacteria in nutritive solutions. A short 

 account of one or two experiments, out of a considerable 

 series, will suffice to show both methods and. results : — 



Exp. I. Five one -ounce phials were charged equally with 

 about a dram and a half of inoculated Cohn's solution, and 

 plugged with cotton wadding. Three were suspended out- 

 side of a window, receiving the direct rays of the sun for 

 the greater part of the day. Of the three, one was wrapped 

 in brown paper, the others left uncovered. One bottle was 

 left standing outside uncovered on the stone windowsill, and 

 one was placed for comparison on a shelf in a tolerably 

 well-lighted room, the sun's rays falling on it for an hour or 

 so in the afternoon. This was on 12th February, the day 



