The Chemistry Of Common Things.

quently , by virtue of their elasticity , occupy a greater space , aud are less dense than at the surface . Tho atmosphere ou the tops of mountains is found to be considerably rarer than at the surface of the earth , the pressure of the superincumbent atmosphere being less in the same degree . On the summit of one of the peaks of tho AndesAvater was observed to boil at 175 ° 37 ° less than its boiling

-, , point at the surface . The pressure of the atmosphere at this point Avas thirteen thirtieths of that at the surface . As the boiling point of A \ 'ater is dependent on the pressure of the atmosphere , a coiwenient means is supplied for ascertaining the heights of mountains , a difference of one degree in this point corresponding to an altitude of about 530 feet . As the pressure of the air is constantly diminishing

from the ground upAvards , it is evident that at a certain elevation the Aveight of the air will be insufficient to counterbalance its elasticity , and this point will consequently determine the boundary of the atmosphere , which is calculated to be at the height of from forty-five to fifty miles . The instrument most commonlin use for measuring tho pressure

y of the atmosphere is the mercurial barometer , which depends on the principle that the atmosphere will always counterbalance a column of mercury equal in Aveight to a column of air of the same diameter , and extending from the height of the mercury to the summit of the atmosphere , This instrument is therefore very useful , in measuring

the heights of mountains , but its principal application is in indicating changes of the Aveather consequent upon variations in the density of the atmosphere . The combustion of bodies in the air is occasioned by the combination of its oxygen Avith the bodies , or with a part of them , the union being accompanied by the evolution of light and heat . If the

air be previously deprived of its oxygen , the combustion Aviil no longer take place , as may be seen by introducing a lighted taper into a . large glass bottle , —it will continue to burn until nearly all the oxygen is consumed , lint a second taper introduced -will be immediately extinguished , the residual air not being able to ^ yield oxygen , and therefore to support combustion . But the oxygen may bo

completely exhausted from the air , and the nitrogen obtained nearly pure by the following process . Place a small piece of phosphorus on a piece of cork floating in a basin of Avater . Light it , and immediately invert over it a wide mouthed bottle , in such a manner that the neck shall dip into the water , and completely surround the cork . As soon as tlie combustion ceases , the bottle will be filled Avith a Avhito substance

resembling smoke , Avhich is phosphoric acid , a compound resulting from the union of the phosphorus and oxygen . On agitating the bottle , this Avhito smoke will disappear , being dissolved by the Avater . The nitrogen ivill be left , Avhich it may be seen cannot support combustion . It is likeAvise utterly incapable of supporting life , and is consequently sometimes called azote , signifying " destructive to life . " Oxygen may be obtained from various substances , but iu the