PUTEEFACTION AND INFECTION. 91 



capacity with infusions of beef, mutton, and turnip re- 

 spectively. The bell-jar was exhausted six times in 

 succession, and filled after each exhaustion with air 

 carefully filtered through cotton-wool. While this air 

 was in contact with the infusions they were boiled in a 

 brine-bath. The receiver was afterwards exhausted as 

 perfectly as a good air-pump could exhaust it; while 

 outside the receiver were hung three tubes to compare 

 with those within. 



Here the protected infusions remained as clear as 

 they were on the day of their introduction, not only 

 after the exposed infusions had charged themselves with 

 life, but for many weeks after they had evaporated 

 away. 



Such, then, are the tests to which I have subjected 

 the statement that 'boiled turnip- and hay-infusions 

 exposed to filtered air, to calcined air, or shut off al- 

 together from contact with air, are more or less prone to 

 swarm with Bacteria and Vibriones in the course of 

 from two to six days.' These results, and others that 

 might be adduced, leave no doubt upon my mind that 

 the deportment of air from which the floating matter 

 has been removed by filtration or calcination is precisely 

 the same as that of air from which the particles have 

 disappeared by self-subsidence. Once really sterilized, 

 an infusion in contact with optically pure air, however 

 obtained, remains sterile. 



§ 21. The Germ-theory of Contagious Disease, 



It is in connection with the germ-theory of con- 

 tagious disease that the doctrine of spontaneous genera- 

 tion assumes its gravest aspect. My interest in the 

 general question was first excited by the investigations 

 of Pasteur, while the medical bearings of the doctrine 



