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scavengers to those microscopic animalculse ; and hence they 

 would tend to diminish the amount of decomposing matters 

 in the atmosphere, rather than increase it. We cannot, 

 however, assign the same office to the various kinds of 

 mould, smut, or blight ; they will grow in the substance, and 

 ultimately perfectly destroy the most healthy parts of plants, 

 and even, in the case of the ergot of rye, will prey upon the 

 vital structure of animals. If, therefore, epidemic disease is 

 produced by the action of mithelium at all, it is produced by 

 that of a similar character to mould and blight; but until 

 we can see the germs of the disease, or are unable to find any- 

 thing more likely to produce it, I think it more rational to 

 adopt the theory — proved certainly to a certain extent, and 

 established by the most conclusive facts — which assigns the 

 phenomena of malaria to putrefactive decomposition. 



Having now given a general outline of what appears to be 

 the cause of epidemic disease in most instances, I will, in the 

 next place, endeavour to point out those causes which appear 

 to me most favourable for its generation and development. 

 One of the most prominent causes calculated to produce 

 disease, or render a large tract of country unhealthy, would 

 arise from want of natural or artificial drainage. All vege- 

 table and animal matters contained in a soil saturated with 

 water, particularly those which, like peat-bogs and marshes, 

 contain a large amount of such matters, will be in a constant 

 state of putrefaction. The compounds produced in them, 

 as in other mixtures, will depend on the temperature of the 

 air, amount of moisture, &c., as above described. It would 

 be almost impossible that from such sources, on some occa- 

 sions, noxious matters would not be given off injurious to 

 life. The position, however, of the undrained land, with 

 respect to river courses, hills, and valleys, would very much 

 modify its sanitary character. A marshy district, surrounded 

 with hills, being protected from strong currents of air, and 



