142 FOREST PROTECTION 



Darkness reduces the damage by SO 2 more than dryness. In the 

 presence of light, heat and humidity, the discoloring and dead- 

 ening action of SO 2 is most intense; which is to say: It is 

 strongest when the vital functions of the leaves are most active. 



Parallel experiments show no discoloration as a consequence of the 

 absorption of SO 2 in the dark room (at night), although such 

 absorption takes place actually. 



Wet leaves show much more discoloration than dry leaves in the 

 same sulphurous atmosphere. 



The main difficulty met in ascertaining the dilution at which SO 2 

 becomes innocuous lies in the disturbing influence of light 

 and moisture. 



After Freytag (experiments in the open air) damage is possible 

 only in humid air, or when the leaves are slightly wet from 

 drizzling rain and from dew. 



Again, after Freytag, air containing less than 0.003% (of weight) 

 of SO 2 is innocuous, even under adverse hydrographic con- 

 ditions and in spite of continuous fumigation, applied during 

 a number of weeks. 



Freytag's experiments are the only open-air experiments which 

 have been conducted with scientific correctness. 



SO 2 and SO 3 are absorbed in the same absolute quantities by the 

 leaves when present in the air in equal proportions. Discol- 

 oration of leaves, however, and decrease in transpiration from 

 leaves are, simultaneously, much smaller in an atmosphere 

 of SO 3 than in an' atmosphere of SO 2 . Consequently, all 

 conditions which favor the formation of SO 3 in the air before 

 the air touches the leaves must decrease the damage espec- 

 ially so in the case of chronic affections. 



The assumption that clouds of smoke interfere with the admis- 

 sion of light and hence with the assimilation of the leaves is 

 erroneous. 



There is no such thing as the "stuffing up" of the so-called stomata 

 found on the leaves (through which inhalation and transpira- 

 tion takes place) caused by soot or solid particles contained 

 in the fumes. 



Experiments made by Stoeckhardt prove this thesis beyond a doubt. 



B. CHEMICAL REMARKS. 



Sulphurous acid (H 2 SO 3) is unknown in the free state; it is likely 



to be contained in the solution of gaseous SO 2 in the water. 

 Sulphurous acid forms primary and secondary sulphites; its salts 



are obtained by saturating a base with a watery solution of SO 2 . 

 If sulphurous acid is eliminated from its salts by the action of stronger 



acids, then it forms its anhydrid and water. 



