DUST AND ITS DANGERS. 49 



teria within it. There are other conditions in 

 which it seems fairly predisposed to their 

 growth and ravages. What the nature of the 

 conditions is, which in one individual or at one 

 time confers immunity to harmful bacterial 

 growth, and at another renders it predisposed 

 to their ravages, we do not know. But very 

 zealous workers are busy with the problem, and 

 we may hope in due time to get light and con- 

 fidence in this obscure field, where we can now 

 but feebly grope. 



