RELATION OF THE DISEASE TO EXTERNAL CAUSES. 51 



found prone to rot : it is, therefore, advisable to keep them 

 in a place which is both cold and dry. 



(199.) Moisture, then, is sufficient to cause local or 

 general death of the potatoe plant. It has not, however, 

 produced the present epidemic, though in solitary cases it 

 may have influenced the progress of the malady. 



OTHER QUALITIES OF THE ATMOSPHERE. WINDS. 



(200.) There are evidently other qualities of the atmo- 

 sphere, besides heat, light, moisture, and electricity. Many 

 persons can tell in bed when the east wind blows ; and 

 such a deleterious influence is exerted by this wind, that 

 every angler knows that he fishes in vain under its pesti- 

 ferous blast. 



(201.) The apparent influence of the east wind upon 

 vegetation is to stunt it, and to arrest the progress of its 

 growth. In this way it may favor the disease to a certain 

 extent : there is not, however, the slightest reason to believe 

 that it actually produces it. The motion or stagnation of the 

 air also influences vegetation : some plants, like fungi, like a 

 close confined atmosphere, others like a more exposed situa- 

 tion. It is also apparent that as the atmosphere contains 

 carbonic acid, the chief food of plants, the health of the 

 plant might be influenced by a proper proportion of that 

 gas. No evidence has, however, been afforded that any 

 variation in amount has existed to cause the present 

 malady. 



(202.) Various hurtful agencies appear to exist which 

 are not referable to any of the above causes ; for instance, 

 the cholera morbus in man seems to arise from some such 



