principle, and that one could be poisoned only by actual 

 contact with some part of the plant. He assumed minute 

 quantities of ^Ilen dust to be in the air to account for 

 the cases of Action J-t a distance" so frequently quoted, 

 Pfaff says: "In my opinion, it is more than doubtful if 

 ever a case of ivy poisoning has occurred without direct 

 contact with the plant or with some article that has been 

 in contact with the plant. The long latent period of the 

 eruption in some oases may obviously render mistakes extreme- 

 ly easy as to the occasion when contact with the plant real- 

 ly occurred," Granting, however, that the active principle 

 is practically non-volatile when isolated from the plant, 

 we cannot say positively that it is not volatile in the juices 

 of the plant, or under the influence of vital forces. It 

 is quite conceivable that the water transpired by the leaves 

 of the plant may carry with it a quantity of the poison suf- 

 ficient to produce the dermatitis on a person very suscepti- 

 ble to its action. It is also conceivable that a volatile 

 poison manufactured by a living plant could become non-vola- 

 tileby changes in it consequent upon thp death of the plant. 



Up to the present time, only three important chemical 

 investigations of the active principle of Rhus toxicodendron 

 have appeared in medical and chemical literature, these 

 being the researches of Dr. J. Khittel, J. i:. Maisch, a 

 ptormacist, and Dr. Franz Pfaff, of the Harvard University 

 Medical School, to whose work reference has been frequently 



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