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prevented from having its atmosphere frequently changed, 

 would become more highly charged with the soluble constitu- 

 ents of the soil than those freely exposed to currents, which 

 would constantly remove it. The condensation of moisture, 

 in the form of mist or fog, appears to be highly favourable 

 to the development, and to increase the intensity, of malaria. 

 It would be likely to do so by repeatedly bringing portions 

 into solution, and thereby increasing and maintaining their 

 decomposition. I believe that the putrid matter, once con- 

 densed and allowed to go on putrefying for a time, would, 

 when again dissolved in air, be more likely to reproduce itself 

 in animal bodies than before. Hence it is that the currents 

 of air which, on still nights, descend down the hill sides, and 

 become cooled so much as to deposit a portion of their mois- 

 ture, in the form of fog in the valleys, or on the course of a 

 river, do not only tend immediately to produce the effects just 

 described, but, by bringing effete matters from a distance and 

 concentrating them on such points, materially increase the 

 action of infectious agents. Valleys and river courses have 

 not only been the prevailing localities of cholera, but of 

 many of the most formidable maladies which afflict the 

 human frame; while elevated situations, from being con- 

 stantly exposed to change of air, and not liable to nightly 

 fog condensations, would be less likely to suffer. Low 

 sheltered localities in towns, such as those so often described 

 by reporters on sanitary matters as centres from which 

 radiate pestilence and poverty in its worst form, and of 

 which we found not a few during our inspection of Sheffield, 

 may be considered to represent small tracts in which in- 

 fectious matters are constantly generated, and for want of 

 a free circulation of air are allowed to accumulate, to the 

 production of disease and death. The inhabitants of every 

 closely built-up court or yard in low localities, where stagnant 

 drains or heaps of decomposing animal matter are allowed to 



