32 NATURE OF CHEMICAL COMBINATIO'. 



tude, and with its distance from the equator. In temperate climates, 

 it oscillates on the same spot between i and li per cent, of the weight 

 of the air ; being least in mid-winter and greatest In the hot months of 

 summer. There are also mingled with the atmosphere, traces of the 

 vast variety of substances which are capable of rising from the surface 

 of the earth in the form of vapour; such, for example, as are given off 

 by decaying animal or vegetable matter — which are the produce of 

 disease in either class of bodies — or which are evolved daring the oper- 

 ations of nature in the inorganic kingdom, or by the artificial processes 

 of man. Among these . accidental vapours are to be included those 

 miasmata, which, in certain parts of the world, render whole districts 

 unhealthy, — as well as certain compounds of ammonia, which are infer- 

 red to exist in the atmosphere, because they can be detected in rain 

 water, or in snow which has newly fallen. 



In this constitution of the atmosphere we can discover many beauti- 

 ful adaptations to the wants and structure of animals and plants. The 

 exciting effect of pure oxygen on the animal economy is diluted by the 

 large admixture with nitrogen ; — the quantity of carbonic acid present 

 is sufficient to supply food to the plant, while it ts not so great as to 

 prove injurious to the animal ; — and the watery vapour sufficas to 

 maintain the requisite moisture and flexibility of the parts of both or- 

 ders of beings, without in general being in such a proportion as to prove 

 hurtful to either. 



The air also, by its subtlety, diffuses itself everywhere. Into every 

 pore of the soil it makes its way. When there, it yields its oxygen or 

 its carbonic acid to the dead vegetable matter or to the living root. A 

 shower of rain expels the half-corrupted air, to be succeeded by a purer 

 portion as the water retires. The heat of the sun warms the soil, and 

 expands the imprisoned gases, — these partially escape, and are, as be- 

 fore, replaced by other air when the rays of the sun are withdrawn. 



By the action of these and other causes a constant circulation is, to 

 a certain extent, kept up, — between the atmosphere on the surface, 

 which plays among the leaves and stems of plants, and the air which 

 mingles with the soil and ministers to the roots. The precise effect and 

 the importance of this provision will demand our consideration in a fu- 

 ture lecture. 



§ 5. The nature and laws of chemical comhination. 



The terms combine and comhination in chemical language have a 

 strict and precise application. If sand and saw-dust be rubbed togeth- 

 er in a mortar they may be intimately intermingled, but by pouring wa- 

 ter on the mass we can separate the particles of wood and leave the 

 sand unchanged behind. So if we stir oatmeal and water together, we 

 may cause them perfectly to mix together, but by the aid of a gentle 

 heat we can expel the water and obtain dry oatmeal in its original 

 condition. Or, by putting salt into water, it will dissolve and disappear, 

 and form what is called a solution, but by boiling it down, as is done 

 in our salt-pans, the water may bo entirely removed and the salt 

 procured of the weight originally employed and possessed of its original 

 properties. 



In none of these cases has any chemical action taken place, or any 



