
Parr] = |. THE ATMOSPHERE. ai 
the earth, the vast beds of coal found all over the world, in geological 
formations of many different ages, doubtless represent so much 
carbon dioxide once present in the air. The chlorides in the sea 
likewise were probably carried down out of the atmosphere in 
_ the primitive condensation of aqueous vapour. It has often been 
suggested that during the Carboniferous period the atmosphere must 
- have been warmer and with more aqueous vapour and carbon dioxide 
in its composition than at the present day, to admit of so luxuriant 
a flora as that from which the coal-seams were formed. ‘There seems, 
however, to be at present no method of arriving at any certainty on 
this subject. 
_ As now existing, the atmosphere is considered to be normally a 
mechanical mixture of nearly 4 volumes of nitrogen and 1 of oxygen 
(N 79-4, O 20°6), with minute proportions of carbon dioxide (carbonic 
acid) and water-vapour and still smaller quantities of ammonia and 
the powerful oxidising agent ozone. These quantities are liable to 
some variation according to locality. The mean proportion of carbon 
dioxide is about 4 parts in every 10,000 of air. In the air of streets 
and houses the proportion of oxygen diminishes, while that of carbon 
dioxide increases. According to the minute researches of Dr. Angus 
Smith, very pure air should contain not less than 20°99 per cent. 
of oxygen, with 0:030 of carbon dioxide; but he found impure air 
in Manchester to have only 20:21 of oxygen, while the proportion 
of carbon dioxide in that city during fog was ascertained to rise 
sometimes to 0°0679, and in the pit of the theatre to the very 
large amount of 02734. Small as the percentage of carbon 
dioxide in ordinary air may seem, yet the total amount of this gas 
in the whole atmosphere probably exceeds what would be dis- 
engaged if all the vegetable and animal matter on the earth’s 
surface were burnt. 
The other substances in the air are gases, vapours, and solid 
particles. Of these by much the most important is the vapour of 
water, which is always present, but in very variable amount accord- 
Ing to temperature.’ It is this vapour which condenses into dew, 
rain, hail, and snow. In assuming a visible form, and descending 
through the atmosphere, it takes up a minute quantity of air, and 
of the different substances which the air may contain. Being caught 
by the rain, and held in solution or suspension, these substances can 
be best examined by analysing rain-water. In this way the atmo- 
spheric gases, ammonia, nitric, sulphurous, and sulphuric acids, 
chlorides, various salts, solid carbon, inorganic dust, and organic 
matter have been detected. To the fine microscopic dust so. abun- 
dant in the air, great importance in the condensation of vapour 
has recently been assigned. (Book III. Part II. Section ii.) 
* A cubic metre of air at the freezing point can hold only 4:871 grammes of water- 
vapour, but at 40° C. can take up 50°70 grammes. One cubic mile of air saturated with 
vapour at 35° C. will if cooled to 0° deposit upwards of 140,000 tons of water as rain. 
Roscoe and Schorlemmer’s “ Chemistry,” i. p, 452. 
