science of 

 Pneumatics ? 



CHAPTETv X. 



PNEUMATICS. 



What is the ^^2. Pneumatics is that department of 

 physical science which treats of the motion 

 and pressure of air,* and other aeriform, or 



gaseous substances. 



Into what uvo . ^^^^ Acriform, or gaseous bodies, may be 



classes may au divided iuto Iwo cUisses, viz., the permanent 



aeriform sub- ' ' ^ 



stances be di- gases, or ihosc wliicii undcr ail ordinary cir- 

 cumstances of temperature and pressure are 

 always in the gaseous state, as common air ; and the va- 

 pors, wdiich may readily be condensed by pressure, or the 

 diminution of temperature, into liquids, as steam, or the 

 vapor of water. 



364. Atmospheric air is taken as the type, or representative, of all perma- 

 nent gases, and steam as the type of all vapors, because these substances pos- 

 sess the general properties of gases and viipors in the utmost perfection. 



What is the ^^^- The atmosphere is a thiu, transparent 



fttmospherc? fluid, Or aeriform substance, surrounding the 

 earth to a considerable height above its surface, and which 

 by its peculiar constitution sup^iorts and nourishes all 

 forms of aniaial or vegetable life. 



• Atmospheric air is conijioseil of oxygen and nitrogen mixed tngether in the proportion 

 of seventy-nine parts of nitrogen and twenty-one of oxygen, or about four-fiftlis nitrogen 

 to one-fifth oxygen. These t\ro gasos existing in the atmosphere are not chemiciilly com- 

 bined with each other, but merely mixed. 



Beside these two ingredients there is always in the air, at all places, carbonic acid gas 

 »nd watery vapor, in variable proportions, and sometimes also the odoriferous matter of 

 flowers, and other volatile substances. 



The air in all regions of the earth, and at all elevations, never varies in compositior, so 

 fai as regards the proportions of oxygen and nitrogen which it contains, no matter whether 

 It le collected on the top of hi;^h mountains, over marshes, or over deserts. 



It is a wonderful principle, or law of nature, that when two gases of different weights, 

 or specific gravities, are mixed together, they can not remain separate, as fluids of differ- 

 ent densities do, but diffuS3 th'-mselvcs uniformly throucrhout the whole sp.ace which both 

 occupy. It is, therefore, by this law that a vapor, arising by it? own elasticity from a 

 volatile substance, is caused to extend its influence and mingle with the surrounding at- 

 mosphere, until its efffcts become so enfeebled by dilution as to be imperceptible to the 

 Bcnsos. Tbus wo are enabled to enjoy and perceive at a di".tance the odor of a flower- 

 Carden, or a perfume which has been exposed in an apartment. 



