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affected by them, whatever protection they may afford to 

 individuals. Should future researches prove these, or any 

 other physical agents, to have specific efficacy in checking 

 the rise and propagation of febrile diseases, beyond what 

 sanitary precautions are confidently expected to accomplish, 

 a great desideratum will be attained; and, in the mean time, 

 my own daily experience warrants me in viewing some of 

 the proposed disinfectants as decidedly beneficial in, at the 

 least, correcting nauseous smells, which I have already shewn 

 to exert a baneful effect upon the nervous system. The 

 difiiculty I have hitherto always found in estimating their 

 intrinsic value in checking the spread of typhus has arisen 

 partly, perhaps, from not having experimented on a suffi- 

 ciently wide scale, but chiefly that their employment has 

 been contemporaneously associated with ventilating and other 

 purifying measures, to which some portion of the attendant 

 success might be fairly ascribed. 



From a very early period the application of caloric with 

 a view to destroy infecting particles, was much esteemed 

 for its efficacy ; and when we reflect on the ventilating 

 properties of heat in altering the densities of the atmos- 

 phere, and thus establishing currents, (as will probably be 

 fully explained to you this day in a subsequent paper), we 

 are not surprised to find that the lighting of large fires in 

 the narrow streets of old towns was a common practice in 

 seasons of unusual pestilence. Heat, indeed, has more 

 recently been suggested as having the power of decomposing 

 the contagious principle ; and analogies are not wanting in 

 support of the proposition, for, inter alia^ it has been found 

 that the vaccine lympth is rendered incapable of communi- 

 cating the cow-pox after exposure to a temperature so low 

 even as 140" of Fahrenheit's scale. This fact accords well 

 with my own theory as to an intimate connection subsisting 

 between aqueous vapour and malarious atoms; for heat would 



