1 6 Dr. G. H. Bailey on 



reliable information that we have is small ; it is, however, 

 increasing rapidly. 



Aitken (Proc. Roy. Soc. Edin. XX. 76) gives results which 

 go far to show that the persistence of town fogs (and it is 

 the persistence which lends them their virulence) is due to 

 the presence of sulphur compounds and mineral matters; 

 and Frankland years ago suggested that the condensible 

 hydrocarbons in air possess a similar property. 



Oliver, in a report presented to the Royal Horticultural 

 Society this year ("Effects of Urban Fog upon Cultivated 

 Plants"), has shown that the presence of as little as 20 parts 

 per million of sulphurous acid will, if the light is also cut off 

 to the extent to which in towns it is cut off, bring about 

 injuries to plants comparable to those which actually occur 

 during fog. Also that mere traces of some hydrocarbons 

 and of pyridine, impurities both found in polluted air and 

 the deposits therefrom, are most injurious. 



Then with regard to human beings, hardly a winter 

 passes without the death-rate from respiratory diseases at 

 times running up to three or four fold the normal. That 

 this is due, in some measure at least, to the abnormal pollu- 

 tion of the air, such as has been indicated, is highly probable. 

 It is a character practically confined to large towns; it is 

 specially characteristic of densely populated districts where 

 pollution of the air is most marked. Doubtless the preva- 

 lence of such ailments is largely affected by climatic 

 conditions, and their seriousness aggravated by a lowering 

 of tone of bodily health already established. 



But this only raises the further question as to how 

 far this very lowering of tone is the result of the 

 constant inhaling of these minimal quantities of sulphur 

 compounds, organic matter, and the like. Even though 

 we are not yet in possession of sufficient information 

 to enable us to speak decisively, there can be little 

 room for doubt that the determination of these minor 



