50
CHAPTER VIII.
THE EFFECT OF SUNLIGHT ON FŒCAL ORGANISM.
It has long been recognized that the action of sunlight has a very powerful:
effect on micro-organisms of all kinds. Consequently, the action of the rays of the sun,
in a tropical climate, is probably one of the main factors that we have to reckon with,
amongst the forces of nature, that destroy bacilli. The recent work by Houston, on
the result of the storage of water, is well known and will be referred to later on.
It was these researches that suggested to us the desirability of carrying out some work
on the action of the direct rays of the sun on fœcal bacilli. It is only possible to
estimate very roughly how great this force is, but we shall show that it is of
enormous importance in the general scheme of nature ; possibly it is the paramount
factor in the destruction of deleterious matter, which is inseparable from the presence
of man and animals.
Sunlight itself is a complex entity, which is made up of rays of different refractive
index, having varying properties, best known to the physicist ; we do not intend to
single out any particular set of rays for study, but to deal with sunlight as a whole.
There is undoubtedly a wide field for research in the study of the action of these various
component parts of sunlight on micro-organisms, but the elucidation of these questions
can hardly be said to fall within the scope of the sanitarian. The work on the action
of sunlight is divided into three separate parts : (1) direct experiment with organisms
by placing them in the sun, (2) special work on the Red Hills Lake by means of weekly
samples taken at various depths, (3) the results recorded from lakes, taken from the
ordinary water samples. This chapter will be mainly occupied with the first of these
heads, namely the experimental work, which we will now consider.
At the commencement of this subject a few preliminary experiments were under-
taken making use of laboratory cultures of organisms. These were mixed with water-
and put in the sun. The results were very irregular, and demonstrated this important
point, that if the investigation was to be of any practical value whatever, we must
use bacilli as we find them in nature, and avoid laboratory cultures. In order to
overcome this difficulty we were obliged, therefore, to forego the use of pure cultures,.
and to work on natural mixtures of organisms, found in the fœces of man and animals.
This, after all, is decidedly a point in favour of the work undertaken. The chief
method followed by us was, to make an emulsion of either cowdung or human fœces in
water, and having placed this in a white porcelain dish, it was placed out in the bright
sunlight, covered with glass to prevent dust and flies, etc., settling in the mixture.
Samples of the contaminated water were examined at intervals. It is true that this
particular method puts the bacilli, so to speak, very much at the mercy of the sun,
but this was unavoidable, as within certain limits, we wished the results to be obtained
with fair rapidity, and although this method is not quite on a parallel with natural.
conditions, it is much nearer to these than would be expected, for in many of our
rivers in India, the water runs over a whitish sand for hundreds of miles in very
shallow streams.
A short series of experiments were carried out before the more systematic work was
undertaken. Several of the results obtained were so interesting that brief mention
will be made of them here. In these experiments a considerable quantity, probably
about a gram of human fœces or calf dung, were mixed with about a litre or a litre
a n d a h a l f o f s t e r i l e w a t e r . T h i s m i x t u r e w a s p u t i n w h i t e p o r c e l a i n p h o t o g r a p h i c d i s h
and placed in the sun. The sediment was not removed by filtration. The depth
of the liquid was not necessarily kept constant. A thermometer was placed in this
mixture to record the temperature ; in no case did it exceed 42° C.-the temperature
of our hot incubator. These preliminary experiments were carried out in September.
5 cc. of this mixture was taken from time to time and run into bile salt lactose broth ;
in some cases the organisms were identified by passing them through the sugars, but
in the majority the result was simply recorded as fœcal bacilli still present in 5 cc .