respiration is concerned, because of the diurnal change in relative humidity, the change usually being inversely as the temperature. The actual amount present, however, does not change greatly from day to night. If therefore, night air is dangerous for convalescents, and it probably is not, it is because of physical and not chemical differences between it and day air.
The importance of ozone as a constituent of the atmosphere is popularly overestimated, and the numerous advertisements referring to it as the basis of the health-giving qualities of the air at certain resorts are largely a delusion and a snare. In a molecule of ozone, one of the allotropic forms of oxygen, three atoms of oxygen are held together in such a way that there is but feeble chemical attraction of two atoms for the third atom, which readily leaves the other two to form a compound with some other element. It is because of the latter characteristic that ozone has its peculiar properties. Though there is considerable diurnal and annual range in the amount present in the atmosphere, and also a large difference between that of the air in cities and that in the country or in the free air, the relative proportion, in general, is but one part in a million. In nature it may be formed (1) by lightning discharges, thus explaining the unusual odor sometimes perceptible immediately after a thunderstorm, (2) by the evaporation of water, particularly in clouds or near waterfalls and fountains, and (3) the action of ultra-violet light upon oxygen, probably most effective in the free air above the highest cloud level. However, the healthful properties of the air at various resorts is due primarily to the dryness of the air, the relatively low temperature with small diurnal and annual ranges, the absence of dust and smoke, and the increased amount of atmospheric electricity, and only secondarily to the larger amount of ozone present in the atmosphere.
Since the sun is ultimately the source of all the heat of the atmosphere the question is sometimes raised: "Why is not the upper air, being nearer the sun during the day, warmer than the lower air, which is more distant; in other words, why is there not an increase rather than a decrease in temperature with height?" Records obtained by means of kites and balloons show, among other things, (1) that up to a height of about 6 miles there is a more or less uniform decrease of temperature with height, (2) that the density of the atmosphere decreases rapidly with height, it being half as dense at a height of 3.5 miles as it is at sea level, and (3) that the water vapor is limited to the lower strata, 80 per cent, of it being below a height of 3 miles. The last two conditions explain the first. Partly because of the adiabatic rate of decrease of temperature of a gas with a decrease of its density, and partly because of the ability of water vapor to remove and to store
