664 
POPULAE SCIENCE EEYIEW. 
tains. The quantity of permanganate may be so arranged as to 
give an approximation to the quantity of organic matter in 
the water when such a quantity of permanganate has been 
added as to afford the slightest possible tinge. If a large 
quantity of permanganate has its colour thus destroyed^ a large 
quantity of organic matter must be present. But for ordinary 
testing it is better perhaps to have a standard water_, — pure 
distilled water_, or some water whose quantity of organic matter 
has been ascertained^ — and then compare the colour it gives 
with the permanganate with the colour given by the suspected 
specimen of water. 
By this test the impurity of water from dissolved organic 
matter can be ascertained with very tolerable accuracy. It 
should^ however, be recollected that it is not applicable to 
water containing iron, as a very small quantity of iron wih 
discolour the permanganate. The quantity of organic matter 
in waters supplied to our great towns differs from one to three 
or more grains per gallon. Suspicion should be aroused when 
the organic matter of water reaches two grains per gallon. 
Having said thus much with regard to the nature of water 
and the means of ascertaining its impurities, the question may 
be still dealt with a little further as to the nature of the evils 
produced by drinking impure water. I have more particu- 
larly referred to cholera, and the evidence of a large number 
of other cases is most convincing as to the production of 
cholera by impure water. It is a question to be determined 
as to whether any specific cholera poison is conveyed into 
water which thus gains access to the individuals attacked, or 
whether the organic matter of the water is in such a state 
as to bring on a state of the system in those who drink it to 
predispose them to receive the poison of cholera from other 
sources. At any rate, our sanitary action in this case should 
lead us to cover both issues of the question, and protect our- 
selves from the dmect poison of the water as well as its pre- 
disposing action. 
With regard to other diseases of the bowels, there is abun- 
dant evidence on record to show that diarrhoea is produced by 
drinking impure water, and that such cases of diarrhoea have 
only ceased when pure water has been had recourse to for 
drinking and cooking purposes. The organic impurities are 
much more likely to produce this effect in warm weather, for 
reasons I have before alluded to ; and this is the reason why 
diarrhoea is so much more frequent in summer, and why water 
should be much more carefully watched in the warm seasons of 
the year. 
Of the various forms of contagious fever which attack the 
inhabitants of this country, there appears to be little doubt 
