SECRETARY'S REPORT. 37 



themselves active disease, than to produce such a state of the system 

 as shall lessen the vital power to resist other causes. Such influences, 

 as they do not produce of themselves specific disease, physicians have 

 properly called remote or predisposing causes, and it is mainly upon 

 their removal that we must depend to diminish the tendency in 

 organic matters to take on disease. 



Other causes, which act either directly or indirectly to produce 

 immediate disease in the previously debilitated tissues of a living 

 body, have received the name of exciting or proximate causes, and 

 may be as various as the circumstances surrounding the various indi- 

 vidual organic bodies which take on diseased action. It is of the 

 latter class of causes only that we propose to speak in this report, 

 reserving the consideration of the predisposing cause or causes for 

 another report, after we have had farther opportunity to trace them. 

 We have been able to trace clearly three distinct causes, which pro- 

 duced immediate disease in the potato, viz. : the atmospheric change 

 before alluded to, after a severe thunder storm, when the leaves and 

 stalks of many fields in a few hours presented a diseased and almost 

 dying appearance, but whether from electrical change, or the sudden 

 abstraction of the caloric from the atmosphere, or from the water 

 applied through means of the shower, we cannot determine. And 

 here let us remark that we have no intention of covering our igno- 

 rance under that convenient scientific cloak, to which many men 

 resort when saying that disease is propagated by the atmosphere, 

 which is only a learned method of saying we know nothing about the 

 subject. 



Another cause which was followed in a few hours by a virulent 

 attack of the local disease of the tuber, was mechanical injury of two 

 kinds, viz. : contusion, as when the tuber was violently struck against 

 a stone, or was brought strongly into contact with another tuber. 

 Having from some circumstances suspected this to be the case, we 

 made some careful experiments, which very clearly revealed the fact 

 that a moderately severe blow by any hard substance, sufficient to 

 leave upon the surface of the tuber the slightest mark, was, during 

 the period while the disease was epidemic, in most instances fol- 

 lowed by the local disease commencing at the bruised spot as a 

 centre. Cutting with the hoe in digging, was in certain varieties 

 almost invariably followed by the access of disease, commencing upon 

 both the divided surfaces, and proceedingrapidly to involve the whole 

 tuber. 



The cause, however, which produced the most malignant type of 

 disease was exposure to the sun for a few hours during the intensely 



