NATURAL HISTORY OF THE BANKS OF THE TAY. 91 
We shall now devote the rest of this paper to an examination of 
the purifying agents which we know to be at work, and endeavour 
to account for the fact that our water is less pure in winter, and 
especially after the early flooding in autumn. 
PURIFYING AGENTS IN THE RIVER. 
I. The action of light as a purifying agent. The water in any 
river is swarming with bacteria, and when sewage is passed into a 
river we are certainly apt to find a larger number of pathogenic 
bacteria present. The excreta from patients suffering from cholera 
and typhoid or enteric fever are especially dangerous, and anything 
which in any way helps to remove or destroy these is most important. 
The bacterial purification of a river has recently been set down to 
“ a process of sedimentation which the micro-organisms in the water 
undergo,” but Buchner in investigations made little more than a year 
ago—see XJeber den Einfiuss des Lichtes auf Bakterien in Centralblatt 
fiir Bakteriologie, vol. IL, 1892—clearly proved that the most im- 
portant factor in the destruction of bacteria in water was the action 
of light. In his experiments he took water highly charged with 
typhoid bacilli, and after exposure to light it was found that these 
were all destroyed. In carrying out his experiments one water was 
charged and exposed to light, and a companion experiment was con¬ 
ducted under exactly the same conditions, with the exception that 
the glass was entirely covered with black paper, so that all light was 
excluded. For example, in one water at the commencement^ there 
were 100,000 germs of the typhoid bacilli in each cubic centimetre 
of the water, and after one hour’s exposure to direct sunlight, not one 
bacillus could be found, while in the control flask a slight increase 
over the 100,000 to start with was got. The experiments with 
diffused daylight were equally satisfactory, only the bactericidal effect 
of the diffused light was less than that of direct sunlight. After 
Buchner’s experiments we have a number of experiments conducted 
by Professor Marshall Ward, F.R.S., of CoopeFs Hill, and communi¬ 
cated to the Royal Society. He conclusively proved that the light 
of a winter’s sun, and also that of the electric arc, rapidly destroy the 
life of the spores of the anthrax bacillus, and showed that the bacteri¬ 
cidal action is really direct, and not due to elevation of temperature, 
or to any indirect poisoning or starving process incident on changes 
in the food materials.” The evidence goes to prove that the effect is 
chiefly due to the rays of higher refrangibility towards the violet end 
of the spectrum—in fact due to the same rays as are most active 
chemically. 
We thus see that one of the most important factors in the purifi¬ 
cation of the Tay from pathogenic bacteria is the action of light, and 
