Air and life / by Henry de Varigny.

  • Varigny, Henry de, 1855-
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Air and life / by Henry de Varigny. Source: Wellcome Collection.

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    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 in 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 <>ood it is immaterial to our senses. This fluid, this gas, may, however, be weighed. Jean Rey1 and Otto von Guericke2 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 (Regnault). 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 Key, 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 has 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.