AND IN EPIDEMICS. 55 



and bloated looks of the white inhabitants of some of these 

 places, evinced the presence of active malaria. 



In vain do we search in the works on received theories, 

 for the cause of this curious influence of night. It is in 

 the day time that evaporation goes on most rapidly, and 

 that chemical changes produced .both by heat and light, 

 are in most active operation. The w^ater is warmer, the 

 common vegetation more vivid, and the great chemist, the 

 sun, is urging on the processes of the laboratory of nature. 

 This is of course admitted by many w r riters, some of whom 

 confess manfully the difficulty of this part of their subject, 

 while others suppose that the miasm evolved during the day, 

 descends at night. Were this really so, it would scarcely 

 account for the extraordinary difference of disease-produc- 

 ing power, between the night and the day; but when we 

 consider how currents of air must sweep away the diurnal 

 emanations, and how late in the night it is before the 

 earth becomes cool enough to detain its proximate atmo- 

 sphere, we can with difficulty admit this mode of explana- 

 tion. 



It has been also said that the baneful effect is due to 

 the great change of temperature which follows the advent 

 of night, by which moisture is precipitated by the air, and 

 the human frame is chilled and sickened. As there is in 

 the most unhealthy regions (coast of Africa), the slightest 

 diurnal change, as rocky islets (De Loss in Africa), are 

 sometimes most pestilential : as a wall, a road, or a screen 

 of trees, sometimes separates a bad from a good locality, 

 and as no such meteorological differences appear to explain 

 the vicissitudes of the health of different years, we must 

 reject such causes, except as excitants of the power of some 

 poison yet to be discovered. 



But when we observe the extraordinary tendency of 



