102 



The Weather. 



mountains,, yet in this thin covering all the wonderful changes 

 of the weather are produced. 



In the early part of the present century Sir Humphrey 

 Davy pointed out among the first, that dry air, at whatever 

 time, or in whatever part of the atmosphere it is collected, 

 consists of nearly twenty- one parts of oxygen and seventy- 

 aiine parts of nitrogen, by volume, in an invariable proportion. 

 This fact was also confirmed by Gay Lussac in his balloon 

 ascents, and is a point of special importance in physics, where 

 the specific gravities of gases are referred to that of dry air as 

 a standard. 



Besides oxygen and nitrogen, the atmosphere contains 

 a very variable amount of aqueous vapour. One and a-half 

 per cent, of this gas, by volume, may be taken as an average 

 proportion between the extreme of dampness and dryness of 

 the air. Carbonic acid gas is another variable constituent of 

 the atmosphere, and its volume, on an average, is little more 

 than a twentieth part of that of aqueous vapour. According 

 to the theory of Dalton, these gases are all mixed together, 

 without any preference among themselves. 



With the exception of hydrogen and ammonia among the 

 gases, and the vapours of a few volatile ethers, aqueous 

 vapour is the lightest known gas. The following table of 

 specific gravities will make this clear. 



Table I. — Specific gravities of certain gases in the atmo- 

 sphere : — 



Hydrogen 

 Ammonia 



0-069 

 0-597 



Aqueous vapour 



0-624 



Nitrogen 



0-971 



Dry air 



Oxygen . 

 Carbonic acid gas. 



1-000 

 1-106 

 1-529 



A mixture of air and aqueous vapour is, therefore, com- 

 paratively buoyant ; but a mixture of carbonic acid and air, 

 or " choke-damp," as it is called, is heavier than common air, 

 and therefore remains in situ at the bottom of shafts and wells, 

 whilst moist air rises to the summit of the atmosphere. 



By an admirable course of experiments at the Royal Insti- 

 tution, Dr. Faraday showed that a number of gases — chlorine, 

 for example, among the elements, and carbonic acid gas among 

 the compound gases — are converted by high pressure into 

 liquids. The liquids again resolve themselves into gases when 

 the pressure is relieved. Aqueous vapour comports itself in 

 every respect like a gas, of which the liquid form is water. 



