These two gases do not exist in chemical combination, but mechanically mixed. This is proved by the fact, that they do not exist in air in the proportion of their combining weights, or any multiple of these; that the proportion varies slightly at different parts; and that the air which is dissolved in water does not contain the nitrogen and oxygen in the proportion 4 to 1 (as in the atmosphere), but in the proportion 1·87 to 1. This means that oxygen, being more soluble in water than nitrogen, has dissolved in a larger proportion; as it certainly would not have done, had the oxygen and nitrogen been chemically combined. The oxygen dissolved in water supplies fishes with the necessary oxygen for their respiratory processes. Similarly the oxygen in the atmosphere is its most essential constituent, being required in all processes of oxidation (_i.e._, combustion), whether in living organisms or in the inanimate world. Nitrogen serves as a diluting agent. It is incapable of supporting life alone; and many of the fatal accidents which have occurred through men descending deep wells without first testing, by means of a lit candle held well below them, the quality of the air near the bottom, have been due to an accumulation of nitrogen in the well. =Ozone= is a condensed form of oxygen, which is present in minute quantities in pure air, and especially during a thunder-storm or after a fall of snow, and in the air near the sea. In it three volumes of oxygen are condensed so as to occupy two volumes. In this condensed condition it has powerful chemical affinities; often oxidising substances which oxygen cannot attack. It is generally absent from the close air of towns and dwelling houses, having been used up to oxidise the organic matter present in these places. Air without it is said to be “devitalised”; and ozone has been described as the scavenger of the air. Ozone can be produced by hanging a piece of moist phosphorus in a room; and it is stated by Dr. Daubeny, that part of the oxygen given out by plants, especially by scented flowering plants, is in the condition of ozone. A small quantity is produced when an electrical machine is worked; its presence is evidenced by a peculiar smell (the name ozone is derived from the Greek word for smell). =Test of Ozone in Air.=—Traces of ozone in air are detected by exposing strips of blotting paper moistened with a mixture of a solution of potassic iodide and starch. If ozone is present, the paper assumes a blue tint, due to the liberation of iodine, and its combination with the starch. Other acid gases may, however, produce the same effect. A second test should, therefore, be tried. Soak red litmus paper with a very dilute solution of potassic iodide, and expose as before. Potassic oxide is produced if ozone is present, and this turns the litmus blue.