HISTORICAL SKETCH 19 



capable of multiplication in the body, and suggested the possibility of 

 their being conveyed from place to place through the air. 



These views, it is true, were largely speculative, and rested upon 

 insufficient experiment; but they were so plausible, and the arguments 

 put forward in their support were so logical and convincing, that they 

 continued to gain ground, in spite of considerable opposition and 

 ridicule, and in many instances the conclusions reached have since 

 been proved to be correct. The fact that infectious diseases were of 

 sudden occurrence, breaking out often in isolated places, and that they 

 frequently remained clinging for long periods to certain localities, 

 leaving others unaffected, was evidence that they were not produced 

 by a gaseous infective agent. Moreover, the mode of infection, its 

 unlimited development among large numbers of individuals, and 

 gradual spread over wide areas the incubation, course of, and resulting 

 immunity in recovery from infectious diseases all pointed to the 

 probable cause being a living organism. 



Among other distinguished men of the day whose observations 

 exerted a most powerful influence upon the doctrine of infection may 

 be mentioned Henle. His writings (Pathological Investigations, 1840, 

 and Text-book of Rational Pathology, 1853), in which he described the 

 relation of micro-organisms to infectious diseases, and defined the 

 character and action of bacteria upon certain phases and symptoms 

 of these affections, are remarkable for their clearness and precision. 



But, meanwhile, the question which most interested these investigators 

 into the cause of infectious diseases was: Whence are these micro- 

 organisms derived which were supposed to produce them? Were they 

 the result of spontaneous generation due to vegetative changes in the 

 substances in which the organisms were found, or were they reproduced 

 from similar pre-existing organisms the so-called vitalistic theory? 

 This question is intimately connected with the investigations into the 

 origin and nature of fermentation and putrefaction. 



Spallanzani in 1769 demonstrated that if putrescible infusions of 

 organic matter were placed in hermetically sealed flasks and then 

 boiled the liquids were sterilized; neither were living organisms found 

 in the solutions, nor did they decompose; and the infusions remained 

 unchanged for an indefinite period. 



It was objected to these experiments that the high temperature to 

 which the liquids had been subjected so altered them that spontaneous 

 generation could no longer take place. This objection was met by 

 Spallanzani by cracking one of the flasks and. allowing air to enter, 

 when living organisms, and decomposition again appeared in the boiled 

 infusions. 



Another objection raised by the believers in spontaneous generation 

 was that in excluding the oxygen of the air by hermetically sealing the 

 flasks the essential condition for the development of fermentation, which 

 required free admission of this gas, was interfered with. This objection 

 was then met by Schulze, in 1836, by causing the air admitted to the 

 boiled decomposable liquids to pass through strong sulphuric acid. 



