Mr. T. Hopkins's Observations on Malaria. 113 



the dew-point should be below the temperature of the atmo- 

 sphere, as that the temperature of the atmosphere should be 

 below that of the human body, unless the temperature be 

 very low. The temperature and dew-point being the same, 

 and at 60°, evaporation would not proceed rapidly from the 

 human body in a stagnant atmosphere. But if a much colder 

 atmosphere, say 'I-O^, were to be fully saturated, the body 

 being 98°, would warm the air immediately around itself, give 

 it a capacity to take up more steam, and at the same time 

 create a more decided upward current that would continually 

 change that part of the air which was in immediate contact 

 with the body. 



Writers and travellers when speaking of a damp atmosphere 

 commonly restrict their remarks to those atmospheres which 

 discharge rain, or are charged with fogs, their object being 

 generally to intimate that such atmospheres wet or moisten 

 substances exposed to them. But an atmosphere may be sa- 

 turated with steam, without giving out any part of it to sub- 

 stances immersed in it which are of an equally high tempera- 

 ture. Suppose both the temperature and the dew-point to be 

 70'^ during the night, neither rain would fall nor dew be 

 formed, and yet with reference to evaporation it would be a 

 damp atmosphere, seeing that no evaporation could take place 

 in it from water of the same temperature. It is the existence 

 of transparent elastic steam in certain quantities in the atmo- 

 sphere which prevents further evaporation from wet surfaces, 

 and it is this steam we have now under consideration, and not 

 either the fall of rain or the floating of condensed vapour. 

 These are effects of this elastic steam having previously ex- 

 isted ; but it is the steam itself of which we are now treating, 

 and of its effects in checking evaporation. 7\.n atmosphere 

 of the temperature of 118°, as found by Oldfield in Africa, 

 might possibly have a dew-point of 98°, and would conse- 

 quently have a drying power of 20 degrees, but the trans- 

 parent elastic steam in this atmosphere would put an entire stop 

 to evaporation from the human body, seeing that the tempe- 

 rature of that body in a healthy state does not rise higher 

 than 98^. It is therefore necessary to observe that it is neither 

 tlie rain that falls, nor the condensed vapour that floats in 

 ilamp chmates where malaria prevails, that is here supposed 

 to constitute that malaria, but solely the quantity of invisible 

 steam which, by its mechanical pressure on the surface of the 

 skin and lungs, prevents the ordinary process of healthful 

 evaporation from being continued, and tlius this invisible 

 steam becomes the real malaria. When this steam stops eva- 

 poration from the body, the 21 \ ounces of water previously 

 Phil. Ma". S. 3. Vol. H. No. 86. Feb. 1839. I 



