318 INTERCHANGE OF ELEMENTS. 



music of the harp of Memnon, awakened by the sun of 

 truth, newly risen from the night of ignorance. 



To return from our long, but somewhat natural 

 digression, to a consideration of the chemical phenomena 

 connected with the atmosphere, and its curious and 

 important element, nitrogen, we must first examine the 

 evidence we have of the condition of the air itself. 



The mean pressure exerted upon the surface of the 

 earth, as indicated by the barometer, is equal to a column 

 of mercury thirty inches high ; that is, the column of 

 air from the surface of the ocean to its highest limits 

 exactly balances that quantity of mercury. If our 

 tube of mercury had the area of one square inch, 

 the columns would weigh fifteen pounds, which repre- 

 sents a pressure of fifteen pounds upon every square 

 inch of the earth's surface. This pressure, it must be 

 remembered, is the compound weight of the gaseous 

 envelope, and the elastic force of the aqueous vapour 

 contained in it.* If the atmosphere were of uniform 



* " This scale, in which the humidity of the air is expressed, is 

 the simple natural scale in which air at its maximum of humidity 

 (i. e., when it is saturated with vapour) is reckoned as = 100, and 

 air absolutely deprived of moisture as = ; the intermediate de- 

 grees are given by the fraction 100 X actual tension of vapours- 

 tension required for the saturation of the air at its existing tem- 

 perature. Thus, if the air at any temperature whatsoever con- 

 tains vapour of half the tension, which it would contain if satu- 

 rated, the degree is 50 ; if three-fourths, then 75 ; and so forth. Air 

 of a higher temperature is capable of containing a greater quantity 

 of vapour than air of less temperature ; but it is the proportion of 

 what it does contain, to what it would contain if saturated, which 

 constitutes the measure of its dryness or humidity. The capacity 

 of the air to contain moisture being determined by its temperature, 

 it was to be expected that an intimate connection and dependence 

 would be found to exist between the annual and diurnal variations 

 of the vapour and of the temperature." Sabine, On the Meteoro- 

 logy of Toronto ; Reports of the British Association, vol. xiii. p. 

 47. The Temperature Tables : by Prof. W. H. Dove ; Reports for 

 1847 should be consulted. 



