64 
PROCEEDINGS OF THE SCIENTIFIC ASSOCIATION. 
collected towards the end of the dry season, when they 
would have acquired the greatest amount of purity, owing 
to the long continued action of filiation through the rocks, 
4c.', and the continued action of the atmospheric oxygen, 
both of which causes would tend to reduce the amount of 
animal and vegetable impurity to a minimum. The pre- 
sence of nitrogen in the form of nitrates and nitrites may 
be due either to the oxidation of animal matter, such as 
manure, &c,, which contains nitrogen, or to washings from 
the strata through which the water passed after its descent 
to the earth in the form of rain. In either case* it is gene- 
rally expressed by English chemists now, by being 
converted into an expression shewing the amount of ‘‘ pre- 
vious sewage contamination, the result being expressed in 
numbers derived from a series of experiments made with 
average London sewage, and shewing the amount of such 
sewage that would be required to produce on oxidation the 
necessary amount of nitrogen in the form of nitrates and 
nitrites. All waters however, even rain water, contain a 
small amount of nitrogen in the form of ammonia and of 
nitric acid derived from the atmospheric nitrogen, so that 
this amount which has been carefully estimated by a long 
series of experiments has to be deducted from the amount 
found in any water before estimating the “previous sew- 
age contamination,’ 7 
The three samples of water I have examined, which are 
proposed to be used for the supply of San Fernando, are — 
1st — from the Hot Springs at Point-a-Pierre — 2nd — from 
the Moriehal Spring at the foot of the Montserrat Hills, and 
the 3rd from the Morocoy Spring near the same place. 
The analyses w r ere performed on very small quantities of 
the water, so that they are not so accurate as I could wish. 
