INTEODUCTORY NOTE. XV 



that learned body, I sought to render their style and 

 logic so clear as to render them accessible to any fairly 

 cultivated mind. The Essays on * Fermentation ' and 

 ^Spontaneous Generation ' have already appeared else- 

 where ; while the first Essay, on ' Dust and Disease,' 

 has been for some years before the public. It may be 

 regarded as a kind of popular introduction to the more 

 strenuous and original labours which follow it. 



The Essay most likely to try the reader's patience 

 is No. III. On the whole, however, and particularly 

 in Its bearings on the Grerm Theory of disease, it is 

 probably the most important of all. The difficulties 

 which sometimes beset the experimenter in these in- 

 vestigations are best illustrated by this Essay. It 

 shows, to my mind in a very impressive manner, the 

 analogy of the spread of infection among organic in- 

 fusions with its mode of propagation among human^ 

 beings. The vital resistance of certain germs to heat 

 is strikingly illustrated in the third Essay, one in- 

 fusion being there proved to maintain its potentiality 

 of life intact after eight hours' continuous exposure to 

 the temperature of boiling water. Under the plain 

 guidance of the Germ Theory, it is however shown that 

 an infusion of this stubborn character may be infallibly 

 sterilized by discontinuqus heating, in one hundredth 

 part of the time requisite when the boiling is con- 

 tinuous. Another question, to my mind of fundamental 

 importance, is also disposed of in Essay III., where it 

 is shown that the £erms which exhibited the foregoing 

 resistance are neither contained in the air, nor attached 

 to the surface of the vessel, above the liquid, but that 



