THE CAUSE OF INFECTIOUS DISEASE. 233 



cation of external energy is necessary in order 

 to do away with the resistance and so bring 

 about the actual fall of the weight just at the 

 right moment. The removal of the resistance, 

 as compared with the actual conversion of the 

 capacity for work into work, is in itself some- 

 thing purely adventitious ; the conversion 

 comes about now easily, now with difficulty, 

 now in this way, now in that, according to the 

 given external conditions. When uncertainty 

 existed about such connection, therefore, the 

 expression " occasional causes " used to be 

 employed. Such modifying conditions are not 

 necessary to the effect, they have not in them- 

 selves the character of true causes, but they 

 are practically important. Corresponding to 

 every form of potential energy there exist by 

 virtue of the external conditions, definite initial 

 circumstances and kinds of resistance which 

 determine whether a given form of potential 

 energy, a given possibility of work, a given 

 internal cause can be converted easily or with 

 difficulty or not at all into kinetic energy, into 

 work, into effect. So long therefore as the 

 external conditions remain the same, the same 

 form of potential energy must be convertible 

 with the same ease into kinetic energy, the 

 cause into the effect. If the conditions change, 

 then the transformation may take place more 



