204 Messrs. A. Downes and T. P. Blunt on [Dec. 19 ? 



appears, therefore, to be a remarkable difference in the rate of action: 

 of light on the germs of Bacteria in water, as compared with its effect 

 on corresponding " germs " in the cultivation-fluid ; insolation of, say 

 a week, accomplishing, in the latter case, what nearly two months 

 failed to do in the former. 



The most reasonable explanation to our minds is the following ; — In 

 Water destitute of organic matter the " germs " are deprived of the 

 nourishment essential for their growth and development — they are 

 starved ; under these conditions their protoplasm reverts to a state of 

 rest and stability, contrasting with that condition of instability which 

 the exhibition of vital energy implies. Possibly they become encysted, 

 the outer portion of the protoplasm being devitalised and protecting 

 the central speck, which may be said to exist rather than to live. 

 When, however, the "germ" finds suitable nourishment, the proto- 

 plasm takes on a higher state of activity and, therefore, of instability, 

 and we believe that this instability of protoplasm favours the action 

 upon it of light, but that in a condition of dormant vitality it is less 

 susceptible. 



Numerous other observations of similar character, which we need 

 not here detail, gave the same results, and the following simple ex- 

 periment, repeatedly confirmed, indicates the germicidal action of 

 light when no water, other than the ordinary moisture in air, is 

 present : — 



April \htli, Four test-tubes are rinsed out with tap-water, inverted 

 to allow the moisture to drain off, and plugged with cotton-wool. Two 

 are covered with laminated lead, and two insolated in the usual way. 

 (Corresponding tubes, charged with Pasteur solution previously 

 sterilised by boiling, speedily became turbid with Bacteria.) 



May 1st. The four tubes are charged with sterilised Pasteur solu- 

 tion. 



In about a fortnight the lead-incased tubes both became turbid,* 

 but the liquid in the insolated tubes was still clear on July 16th. 



We now proceed to give an account of experiments which bear 

 more directly on the intimate nature of the action under consideration. 



From an early stage in the investigation we felt that the best way 

 of approaching the problem was by examining the behaviour of organic 

 bodies generally when exposed to sunlight. Taking, in the first in- 

 stance, the comparatively simple molecule of oxalic acid as the subject 

 of our experiment, we found that a decinormal solution ('63 per cent.) 

 was entirely decomposed by sunlight. It was obviously important 

 to ascertain what was the nature of the decomposition, i.e., whether it 

 were a disintegration of the molecule into water, carbonic acid, and 



* Subsequent microscopical examination showed that this "was due in the one case 

 to a species of Sarcina, in the other to Bacteria, which did not, however, take on a 

 very vigorous development. 



