1894.] Professor Ward on the Action of Light on Bacteria, dc. 259 



WEEKLY EVENING MEETING, 

 Friday, April 27, 1894. 



Hugo W. Muller, Esq. Ph.D. F.K.S. Vice-President, in the Chair. 



Professor H. Marshall Ward, D.So. F.R.S. F.L.S. 



Action of Light on Bacteria and Fungi. 



The Italians have a proverb which runs, " Dove non va il sole va il 

 medico " — Where the sun does not enter the doctor does — and which 

 may be taken as an expressive summary of the experience of a people 

 who have had opportunities of learning by many and varied trials how 

 important for health is a due exposure to sunlight. Moreover we find 

 the same proverb, in almost the same form, in the Provencal dialect 

 — " Di lo inesou ente nentro pa lou sonle riebo lou medeci " — and that 

 this empirical recognition of the sanitary powers of sunshine is 

 common to many nations, and every one who has resided in India and 

 the East knows how naturally the servants expose clothing and other 

 articles to the direct rays of the sun. 



In answer to the question, "Does the above proverb, and its 

 equivalents in the languages of other peoples who have much to do 

 with intense sunshine, really imply any knowledge or suspicion of 

 the direct action of solar rays on organic materials or objectionable 

 living beings? we are no doubt justified in replying, No. If 

 they have thought about the matter at all, the people have assumed 

 that the heat of the sun's rays and the ordinary action of the air are 

 the factors concerned ; and that it is no doubt a sort of mixed effect 

 of dryness and ventilation that is to be aimed at when they expose 

 the interiors of dwelling rooms, soiled clothing and so forth to the 

 process of being " aired," as they have it. 



On the other hand, we find that experience of similarly varied and 

 often bitter kind has also taught the children of sunny climates that the 

 very brilliant sunshine which they so correctly extol as efficient in 

 purifying their houses and apparel is a dangerous enemy to human 

 beings who are unduly exposed to the direct solar rays. All the 

 painful experience connected with snow-blindness, sunburn and sun- 

 stroke may be invoked in support of this statement, and it requires 

 but a careful study of this subject to be convinced that in these cases 

 at any rate the effects cannot be put down to the mere heating power 

 of the sun's rays. 



Closer examination of the whole question drives us to the con- 

 clusion that the effects referred to, and many other effects of direct 

 insolation, are not due to the heat rays at all, but to actions of quite 

 other kinds induced by rays long unsuspected of having anything 



