40 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY- 



white flowers I have already remarked that some are affected by 

 whilst others resist fog. We have, then, indication that the 

 organic impurities of fog are not unconnected with the injuries 

 produced ; an indication which is strengthened when we consider 

 the action of the bodies cited. These not only produce a similar 

 reaction, where a reaction occurs, but also fail to call forth 

 the characteristic colour-change in one conspicuous group— the- 

 bulbous Monocotyledons — which likewise resist fog. I am far 

 from asserting that the reagents I have employed are the par- 

 ticular impurities which we have to deal with in fog, though I 

 suspect these impurities to be substances of the same character 

 and general constitution. Nor must it be thought that I am toa 

 prone to neglect the toxic action of sulphurous acid. This no- 

 doubt, in all cases, takes a chief part in the destruction (i.e. 

 killing) of the protoplasm, whilst the action of the accompanying 

 bodies comes out in the secondary changes — the yellowing and 

 browning to which allusion has been made. 



But until our knowledge of the volatile tarry and other 

 organic substances in [the air is much extended ; until fog 

 ceases to be a mixture of a limited number of known with an 

 almost unlimited number of unknown compounds, we must be 

 content to leave the problems discussed in the report in large- 

 degree unsolved. 



IV. The Changes which the Chlorophyll undergoes in 

 Leaves injured by Fog. 



In view of the sensitiveness of chlorophyll to the action of 

 acids, it might be supposed that this pigment would serve as a 

 ready indicator of the presence of acid fumes in the atmosphere- 

 But although sulphurous acid attains to a recognisable per- 

 centage in fog, spectroscopic examination of the alcoholic ex- 

 tracts of injured foliage gives altogether conflicting results. 

 Without going into details, it may be stated that the samples 

 which were from time tojtime submitted to Dr. Schunck, and 

 upon which he has kindly reported, fall into two categories :— 

 (1) Those in which the chlorophyll was unaltered ; (2) those in 

 ■which there were indications of the presence of an acid which 

 had caused sometimes a slight alteration of the chlorophyll, 

 in others a more complete destruction of the chlorophyll, leading 



