12 Dr. G. H. Bailey on 



air, comparatively little has been done in the direction of 

 such matters as hydrocarbons, sulphur compounds, and 

 organic matter, &c, occurring in polluted air in larger 

 quantities and in country air in traces. 



This, no doubt, partly arises from the analytical 

 difficulties presented by the problem, but chiefly, I think, 

 from a want of appreciation of the importance of such 

 determinations. For though it would be freely acknow- 

 ledged that such bodies as sulphuretted hydrogen, sulphurous 

 acid, pyridines, and the like, are, in a concentrated form, 

 injurious to plant and animal life, it is usually taken for 

 granted that a dilution to such an extent that we have to 

 speak of parts per million, must rob these substances of any 

 deleterious character which they possess in a concentrated 

 form. It is the purpose of this paper — 



(i) To suggest the determination of certain minor 

 constituents of air as a means of discriminating between 

 polluted and unpolluted air, and as a measure of pollution. 



(2) To bring forward evidence that, minimal though the 

 quantities may be, they are, notwithstanding, sufficient 

 to bring about serious disorganisation in plant life and in 

 human beings. 



So far as our present knowledge goes, the more important 

 of the minor constituents of town air, excluding carbonic 

 acid gas and moisture, may be classified under the following 

 heads : — 



Solid matter. — Carbon, associated with which is a 

 considerable quantity of sand and some iron and oxide 

 (probably sulphide) of iron ; mineral salts, such as sulphate 

 and carbonate of ammonia, common salt, and in manufac- 

 turing districts products ejected from works, especially 

 alkalies, copper salts, and the like ; organised substances and 

 micro-organisms. 



Liquid matters. — These are hydrocarbons, pyridine, free 

 sulphuric and sometimes nitric acid. 



