CARBON DIOXIDE. 27 



ties whatever, that is, it appears to be incapable of uniting with 

 any other substance or even with itself, for unlike most gases, 

 its molecule contains only one atom. Its density when care- 

 fully purified was found to be 19*957, which gives as its 

 atomic (and molecular) weight the number 39-914. So far as 

 is known argon takes part in none of the chemical changes 

 which occur in the atmosphere. 



Associated with argon, Eamsay has found several other 

 gases of the same inert character and monatomic molecules, 

 but differing in density and other physical properties. These 

 have been named helium (identical with the substance so 

 named, which was, long ago, detected in the sun by spectro- 

 scopic analysis), density 2 - 0; neon, density 1OO ; krypton, 

 density 22'5 ; and xenon, density probably about 40 41. 

 These elements, however, are present in such extremely small 

 quantities and their chemical inertness is so great that they 

 are probably of no importance from our present standpoint. 



The amount of argon present in the air is apparently very 

 constant about '94% by volume or 1*3% by weight. It con- 

 stitutes about 1-19% by volume of the residue left after the 

 removal of oxygen. 



Carbon Dioxide. This constituent, though present only in 

 small proportion (usually less than '04% by volume), is of great 

 importance with reference to vegetable life. Its amount is 

 subject to considerable variation since it is a constant product, 

 in large quantity, of the combustion and putrefaction of all 

 organic bodies. 



In the free open country, air contains on the average about 

 033 % of carbon dioxide/'' In large towns or in the neighbour- 

 hood of manufactories the quantity is usually larger. In 

 London, Angus Smith found -044, in Glasgow '05, in Man- 

 chester -045. The amount is always greater during fogs 

 (sometimes rising to '!%) and snow r . 



Aii 1 in the country shows a distinct diurnal variation, the 

 amount of carbon dioxide being greater at night. Armstrong's f 

 experiments made at Grasmere in summer-time gave -0296% 



* Recent determinations have almost invariably given lower values for the mean 

 amount in the atmosphere, 

 t Pro. Roy. Soc. 1880, 343. 



