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discredit this hypothesis. There can be no doubt that as 

 much mystery hung over the cause of the potato disease as 

 over the phenomena of malaria ; yet it is found to arise from 

 a microscopic plant, whose habits appear to be similar to 

 those producing mould or blight. The peculiar, and gene- 

 rally fatal disease induced in the human subject by eating 

 rye bread, or using rye meal, containing portions of the 

 ergot, has been clearly proved to arise from the rapid 

 growth and extension of the mithelium, which produces the 

 disease in the grain. It is true that the microscope has 

 been directed in vain to discover these germs of fever in 

 diseased subjects, and also in the putrid liquids condensed 

 from infected air ; but with our knowledge of the limits of 

 the power of the microscope, we may conceive vital germs 

 too minute for its reach ; and consequently we must not 

 altogether on this account discard an hypothesis which has 

 certainly many important facts in its support. Probably the 

 progress of the malignant cholera and some other epidemics 

 could be much better explained on the mithelium hypothesis 

 than on the putrefaction one ; yet whether we believe it to 

 arise from organic growth or decomposition, we must always 

 bear in mind that the latter is invariably attended with the 

 development of the former. In every drop of stagnant water 

 — in every heap of decomposing rubbish, myriads of micro- 

 scopic germs of life are developed and subsist ; they find in 

 it the food necessary for their growth, and the conditions 

 necessary for their reproduction. It appears to me more 

 rational to assign their existence to the peculiar state of the 

 decomposing fluid, and not the state of the fluid to their 

 existence. The various kinds of mould are only produced 

 where they find the necessary conditions for their develop- 

 ment. The yeast plant is not the cause of the decomposition 

 of the yeast, but lives in consequence of its change. More 

 than one microscopist has assigned the oflftce of nature's 



