To revert now to our atmosphere, there is another interesting point which requires our notice; namely, are the constituents of this atmosphere chemically united together, or only simply mixed in certain proportions? We believe that it consists of a simple mixture only of two essential gases, or elements, namely, oxygen and nitrogen, with a small and variable quantity of carbonic acid, and also with water in a state of vapour. We may consider the last two as accidental ingredients, essential as the vapour of the atmosphere may be to the necessities of animals and plants, to luxuriance of scenery, and fertility of soil. The essentials of air are united in the following proportions, namely, one part of oxygen and two of nitrogen, or one atom of oxygen and two atoms of nitrogen; but as the atomic weights of oxygen are as 8 to 14, the proportion of the weights of the two in any given quantity of air will be that of 8 to 28, or two to seven; in other words, nine grains by weight of air will contain two grains of oxygen, and seven of nitrogen, supposing the air to be pure. But, as we have said, a portion of carbonic acid gas is usually contained in air; this gas is exhaled from the earth, and collects in certain localities, rendering the air more impure than it otherwise would be; as a rule, however, the quantity of this gas in intermixture with the air we breathe--its lower strata--varies from three to eight parts out of a thousand in weight; in its pure state, it is immediately destructive to animal life; but thus diluted is in this respect innoxious, while to the vegetable kingdom it is necessary. With respect to water in a state of viewless vapour, the quantum in any given weight of air seldom exceeds 1-1/2 per cent.