162 WATER, AND ITS IMPURITIES. 
But there are certain circumstances which are closely con- 
nected with the kinds as well as the quantity of organic life pre- 
sent in water, beyond those already noticed ; these are, free 
exposure to air, light, and warmth, which all greatly favour de- 
velopment, and so assist in determining the kind and amount 
actually present. 
For the purpose of shewing clearly the influence of these 
agents, we instituted a considerable variety of experiments ; as, 
however, the powerful operation of the causes referred to are 
well known and undisputed, it is unnecessary to detail their 
nature. 
Now rivers, reservoirs, and other large bodies of water are 
exposed to the influences alluded to, whereby the water con- 
tained in them is generally deteriorated. 
We have dwelt thus long and fully on the organic impu- 
rities of water, because of their extreme and primary importance, 
for it is on these that the deleterious properties of impure water 
for the most part depend. 
Until very recently, chemists did not in general attach suf- 
ficient importance to these organic contaminations, and in most 
of their analyses we find the different kinds of organic matter, 
vegetable and animal, living and dead, all lumped together, and 
included under the word “ Traces.” 
Indeed chemistry is but ill-adapted to investigate the nature 
of these organic matters; it gives but a very rough estimate 
only of their gross amount, and does not discriminate, as we 
have said, the animal from the vegetable, the dead from the 
living, and tells nothing about the families, genera, and species, 
to which the numerous living productions contained in impure 
waters severally belong, or of their habits and modes of life, &c. 
This inquiry belongs to the naturalist, the physiologist, and 
the microscopist, and to Dr. Hassall is due the honour of having 
first applied the resources of these extensively, and in a practi- 
cal as well as a scientific manner, to an examination of the 
actual condition of water in general, and particularly the state of 
that now in use in the metropolis.* 
Uses of Vegetables and Animals in Impure Water. 
In the existence in impure water of different kinds of organic 
productions, we recognise the fulfilment of wise and beneficial 
purposes. 
* Results of a Microscopical Examination of the Water supplied to the 
Metropolis and the Surburban Districts. — The Lancet, March 1850. 
A Microscopical Examination of the Water supplied to the Inhabitants of 
London and the Suburban Districts, Illustrated with coloured plates. Samuel 
Highley, Fleet-street. 
Evidence before the General Board of Health. Report, Appendix III. 
