200 Mr. Ivory o?i the Constitution of the Atmosphere. 

 because this assumption conjoined with the condition, 



V = y + v' 

 will determine wand u', when V is given: thus V = 100; 

 V = 20'9, o' = 79"0. From the foregoing formulas we like- 

 wise obtain D = R; that is, the density of the mixture under 

 the pressure j) and temperature 6 is equal to the density of 

 atmospheric air under the same pressure and density ; which 

 proves that the mixture can be no other than atmospheric air. 



What has been demonstrated as necessary consequences of 

 the physical properties of the gases agrees with the results of 

 many laborious experiments undertaken for the purpose of 

 resolving air into its constituent elements. Whatever be the 

 local circumstances in which air is collected, it is found, on 

 being analysed, to consist in volume of 21 parts of oxygen and 

 79 parts of azote. This is true not only at the earth's surface, 

 but on the tops of elevated mountains situated at great di- 

 stances from one another, and even at the greatest height to 

 which man has been able to ascend in the atmosphere, as is 

 proved by the analysis of the air brought down by Gay- 

 Lussac in his aeronautic ascent from a height of about 6400 

 yards. 



Another observation it is important to make. The law of 

 Mariotte has been verified in the case of atmospheric air, be- 

 tween very wide limits of pressure and temperature: and as 

 it follows from what has been shown, that this law, and the 

 formation of air by the combining of two gases in a constant 

 proportion, are reciprocally consequences of one another, we 

 have two independent experimental researches confirmatory 

 of those co-existing properties of air. It appears, therefore, 

 that there is good evidence to conclude that every portion of 

 the atmosphere from the earth's surface to the top, is com- 

 posed of two volumes of oxygen and azote in the proportion 

 of 21 to 79, and that the law of Mariotte is applicable at all 

 heights. 



The atmosphere at any two points on the earth's surface, 

 being composed of the same elements combined in the same 

 proportion, can differ from one another only on account of 

 the heat derived from the earth or from other sources. The 

 effect of such heats is not to alter the aerial fluid in its con- 

 stitution, but merely to dilate it, and to vary the height at 

 which a given temperature will prevail in the column of air. 

 It is therefore necessary to investigate the law according to 

 which the heat decreases in ascending, in order to complete our 

 knowledge of the atmosphere. Little progress has been made 

 in this research. The measurement of small heights leads to 



