292 GERWS. 



medium size. While of the still smaller varieties the common 

 bacillus of cholera and the rod-shaped curse of consumption have 

 become household words during the last ten years, and it is 

 known that the souring of milk is produced in a similar way. It 

 was more particularly about these minutest of living plants that 

 are to be found on almost any coin we have in our pockets that 

 my lecture was intended to speak ; and the main idea one has to 

 bear in mind is that these bacteria being, like the white potato 

 seedling, unable to make use of the carbonic acid in the air, are 

 driven to seek for the carbon they require in some animal or 

 plant which may be either living or dead. In doing this they fre- 

 quently, but by no means always, manufacture certain chemicals 

 which act in a poisonous way on their host, producing according 

 to the nature of the bacterium (minute white plant) a definite 

 disease. 



These bacteria are exceedingly small, in many cases one hun- 

 dred million would have to be laid side by side to make a square 

 inch of film, which was but one ten-thousandth of an inch in 

 thickness. Their minute size, coupled Avith the fact of their 

 being universally present wherever dust makes its appearance, 

 made them the last battle ground of the believers in spontaneous 

 generation or the theory of life springing from dead matter. 



A most interesting account of the theory may be read in Tyn- 

 dall's " Floating Matter of the Air," where the author traces 

 the idea backwards to the ancient times when the Britons, seeing 

 a moist ditch in spring teeming with tadpoles, accounted for their 

 presence by the belief in the spring sun having power to warm 

 the dead mud of the ditch into a wriggling swarm of life, just in 

 the same way as the Egyptians relegated the power of forming 

 eels to the slimy oose of their own river, the Nile. 



The method of preserving meat in tins is one of the outcomes 

 of this possibility of preventing the formation of bacteria, and 

 the belief that life only comes from life. If meat is left in con- 

 tact with the ail-, it is a well-known fact that especially under 

 suitable conditions of temperature, as in summer, it will very 

 quickly go bad, the cause being a particular kind of bacterium. 

 Now if these latter can spring spontaneously from meat which 



