138 AIR AND LIFE. 



I. — Air Considered from the Physical Point of View. 



Vast as are the proportions of the atmosphere it is none the less invis- 

 ible. It surrounds us on every side; we are bathed m it, and we do 

 not see it; when it is not in motion we do not feel it. Although having 

 material existence and creating material effects, for evil or for good, it 

 is immaterial to our senses. This fluid, this gas, may, however, be 

 weighed. Jean Eey 1 and Otto von Guericke 2 were the first who gave 

 positive proof of this, and showed that a glass receiver in which a 

 vacuum had been made — even imperfectly — weighed less than the same 

 receiver in normal and free connection with the surrounding atmos- 

 phere — that is, full of air — and, on the other hand, a receiver into which 

 air is forced and maintained under pressure weighs more than the 

 same receiver full of air at the normal pressure pro loco et tempore. 

 One liter of air, pure and dry, under the pressure of 760 millimeters, 

 at 0° temperature, and at the latitude of Paris, weighs 1.293 grams 

 (Kegnault). It weighs more if the pressure is higher, less if it is 

 lower; and hence a liter of air has more weight at the bottom of a 

 shaft in a mine than at sea level, and less on top of a hill or moun- 

 tain. The higher the altitude at which air is weighed the less it 

 weighs, because it expands, the same weight of air occupying a larger 

 space or volume. Air is more dense at low stations, less dense in the 

 higher strata of the atmosphere, so that when the weight of air is men- 

 tioned it is always given with reference to a certain altitude, to a cer- 

 tain pressure, and also to a certain temperature and hygrometric state, 

 because these different conditions exert a considerable influence upon 

 the matter. 



As in the case of other gases, air is made up of molecules, and these 

 are considered as being in a state of perpetual motion. It has been 

 reckoned that the number of impacts or collisions to which each mole- 

 cule is subjected during each second, in the tremendous turmoil which 

 takes place in the air, amounts to something like 4,700,000,000 ! These 

 molecules are exceedingly small, and Sir William Thomson (Lord 

 Kelvin), Clerk Maxwell, and Van de Waals give their dimensions as 

 being less than a fraction of one-millionth of a millimeter, 1 cubic cen- 

 timeter of air, at 0° and 760 millimeters pressure, containing, in round 

 numbers, some 21,000,000,000,000,000.000 of these molecules. 



1 have referred to the fact that the weight of the air is not the same 

 in all localities. It also varies in the same locality. The sum total of 



! Jean Rev, French physician and physicist, said, in 1630, that if tin is burnt in 

 contact with air, it increases in weight, and this increase is due to air which lias 

 been absorbed by the metal during the combustion. 



2 Otto von Guericke, born 1602, died 1686. He also demonstrated atmospheric pres- 

 sure by means of the instrument called the Magdeburg hemispheres — two hollow 

 hemispherical cups which it is very difficult to separate when a vacuum has been 

 created in the interior. 



