44 TraTisactions of the 



ment of height by its means ; the decrease of density due to 

 elevation, and the reduction of temperature caused by expansion; 

 and, with a description and explanation of the colour of the 

 atmosphere, and some statistics regarding its weight, specific 

 gravity, specific heat, &c., the first part of the paper was concluded. 

 In considering its chemical constitution, he noticed the ancient 

 idea of the atmosphere's elemental character, and sketched the 

 early history of the discovery of its compound nature, the grave 

 doubts entertained as to whether its constituents were simply 

 mechanically mixed or chemically combined, owing to the remark- 

 able constancy in composition. The normal composition was then 

 given, and the various functions of each of these pointed out : 

 oxygen, the supporter of respiration and combustion ; nitrogen, a 

 dilutant of its more active associate ; carbonic acid, nourishing 

 the vegetable kingdom, the oxygen being at the same time set 

 free for the support of animal life ; aqueous vapour, the source 

 of dew, fog, cloud, rain, snow, and hail. The following is an 

 extract : — 



' It is but a fog — a large aggregate of vesicular and solid par- 

 ticles of water, in the far depths of the air, that produces what we 

 call clouds ; which as they loom darkly o'er this world of ours, in 

 all their fantastic shapes, mysterious, terrible, seem to bar our 

 way to the bright heaven they hide from our view ; or again, 

 when the noonday sun lights up their thousand forms with 

 brilliancy divine ; when the crimson glow of the evening trans- 

 forms them into a mass of blazing radiance, they seem 



" As though an angel host in upward flight 

 Had left their mantles floating in mid-air." 



Wonder upon wonder meets our eye ; now we see among their 

 giant forms a snowy mountain hanging in mid-air; now a toppling 

 crag with dark seared sides; now again a summer landscape fading 

 away in the far distance, so sweet, so beautiful, that we can almost 

 fancy it to be the path which leads us upward to our God above, 

 when once the icy river of death be past ; or again, a depth of 

 impenetrable blackness, a very chaos, that might fitly be the 

 portals to the domains of the arch-fiend himself/ 



The abnormal constituents of the air were then briefly noticed: 

 oroanic matter and germs, the propagators of disease ; nitric acid 

 and ammonia, the chief source of nitrogen for plants ; carburetted 

 and sulphuretted hydrogen, sulphurous and sulphuric acids, the 

 products of the combustion of coal and decomposition of animal 

 ujatter ; selenuretted hydrogen, said to be present in dry-fog ; and 

 ozone, the great purifier of the atmosphere by destroying mias- 

 mata and organic particles with its powerful oxidizing properties. 



