OP MALARIA. 15 



A third party refers all cases of periodical disease exclu- 

 sively to sensible changes, and thinks the proximity of a 

 marsh only efficient as presenting an evaporating surface, 

 by which the air is made colder and damper. 



Dissatisfied for many reasons, to be hereafter offered, 

 with the vegetable theory, and with the evaporating theory, 

 and indeed with the hypothesis by which both are united, 

 authors of our own time have suggested a variety of ex- 

 planations, which it may not be inexpedient to pass in 

 cursory review. 



The commonly received marsh theory is well stated and 

 supported by MeCulloch, to whose work on malaria I re- 

 fer you for a view of that side of the question. It is, in a 

 much more masterly and precise manner, sustained by Di% 

 Craigie, of Edinburgh, to whose volumes on the Practice 

 of Medicine, you may most profitably resort for a learned, 

 lucid and, I think, impartial array of the facts and opin- 

 ions bearing on that side of the question. MeCulloch in- 

 volves himself in difficulties without seeming to see them, 

 whilst Craigie, although inclined to the same conclusions, 

 views with a master's eye, the whole of the impediments 

 and objections. The objections presented by the latter 

 are: the low temperature at which these disease-producing 

 changes may take place ; the unaccountable production 

 of them in places where there is no apparent vegetation 

 and often no marsh ; the exemption of certain places where 

 occur all the seeming elements of decomposition ; the inex- 

 plicable effects of rural cultivation ; and the unexplained 

 vicissitudes of health in the same places in different though 

 similar years. 



Denying the vegetable theory, and indeed assuming the 

 position that we are as yet totally ignorant of the nature 

 and true source of the cause of malarious fevers, my emi- 



