96 A TRACT OF MISCELLANIES. 



at once make an invasion upon all the families of 

 the district over which it extends, but capriciously, 

 as it appears to us, to attack isolated families, while 

 others, who seem equally exposed to the contagion 

 who live in the same district, or in the immediate 

 neighbourhood, escape ? 



159. Do isolated swarms of insects exist in the 

 atmosphere analogous to the animalcules that exist 

 in water, but which are so minute as not sensibly to 

 impair the transparency of the atmosphere ? * If so, 

 are those insects, during the process of respiration, 

 largely inspired into the lungs, insomuch that a mor- 

 bid taint is imparted to the blood ? and is it from 

 this cause that fevers and those other diseases which 

 are supposed to be atmospheric, have their origin 

 one species of insect as it exists in the atmosphere 

 generating one kind of fever other species generat- 

 ing fevers of a different kind, and so on with other 

 diseases which are also supposed to be atmospheric ? 



160. If animalcules exist in the atmosphere as 

 well as in water, then as putrid water is the habitat 

 of the one class, may not a putrid vapour existing in 

 the atmosphere be the habitat of the other ; and is 



* Ehrenberg has discovered in bog-iron ore, fossil animalcules 

 so very minute, that a cubic inch of the ore contains two millions 

 of millions of those animalcules. 



