purify the blood, and give it afresh all the conditions of life and growth.

Large amounts of carbonic acid are thrown locally and rapidly into the atmosphere by the respiration, and still more by the combustion, of a large city; large amounts are also taken locally and rapidly from the air, as vegetation spreads over large areas in spring, or grows rank in early summer. The atmosphere has sufficient volume not to be affected in its proportions by these quick changes, though carbonic acid constitutes but one part in twenty-five hundred, while the law of diffusion among gases immediately and fully corrects the local deficiencies or accumulations of any one ingredient.

The atmosphere has also a most important office in the diffusion of light. The world is filled with light, its rays moving in all directions with comparative equality. This even, soft light is due largely to the reflection and dispersion of its rays by the atmosphere, with its floating air-dust. Instead of intense cutting beams, coming direct from the sun, we have a pervasive light, that presses in from all quarters, and goes out at all quarters. Though the sun is the source of light, its rays are quite another thing as filtered, reduced and dispersed by the atmosphere, from what they are beyond its range. In like manner, but more directly and exclusively, the atmosphere is the medium of sound. The world would not merely be comparatively dead to motion, it would be quite dead to sound, were it not for the air. Its density determines the presence and quality of all sounds.