On the Bain and Drainage -Waters at Rothamsted. 249 
The agreement between the amounts of rain actually recorded 
and those calculated by the ])ractical rules above referred to is 
throughout very fair, and amply justifies their application to 
purposes of water-supply. 
o. The Composition of the Rain-water. — When the vapour of 
water is condensed in the upper regions of the atmosphere, 
and descends in the form of rain, hail, or snow, it reaches 
the earth holding in solution more or less of the gases present 
in the atmosphere. The quantity of any gas dissolved by rain 
will depend on the solubility of that gas in water, will be greater 
in proportion to the abundance of that gas in the atmosphere, 
and will also be greater, other circumstances being equal, as 
the temperature of the rain is lower, and the pressure of the 
atmosphere higher. In rain-water collected in the country 
nitrogen and oxygen will be the gases chiefly present, with a 
small quantity of carbonic acid, and a still smaller amount of 
carbonate of ammonium. 
Besides the gases which rain holds in solution, it contains 
various solid substances gathered from the atmosphere during its 
descent. Some of these, as the chlorides, sulphates, and nitrates 
of sodium, calcium, and ammonium, are dissolved by the rain ; 
others, as particles of dust and soot, are merely mechanically 
held, and give to rain-water its ordinary dirty appearance. 
Most of the constituents of rain-water are present in very minute 
quantity, and the powers of chemical analysis are taxed to the 
utmost to determine them. 
It will be well to notice as briefly as possible the sources 
of the more important matters dissolved by rain-water in its 
passage through the air. 
The ammonia of the atmosphere is derived from the decay of 
animal and vegetable matter, both on land and in the ocean, 
and from the combustion of fuel, especially coal ; the air of 
towns is much richer in ammonia than that of the country. 
According to M. Schloesing, the ocean of the tropical regions is 
the most important source of atmospheric ammonia. At the 
high temperature of tropical latitudes, the ammonia produced by 
the decav of organic matter diffuses freely into the atmosphere, 
and is carried by winds to all parts of the globe. In northern 
latitudes southerly winds are those richest in ammonia. 
The nitric acid present in the atmosphere is due in part to 
electrical agency. Discharges of electricity in the air deter- 
mine the combination of the nitrogen and oxygen of which the 
atmosphere is composed, nitrous acid being formed ; ozone is at 
the same time produced, which is capable of oxidizing both 
nitrous acid and ammonia, nitric acid in each case resulting. 
A source of nitric acid independent of electrical discharge, 
