-1859] WRITINGS OF JOSEPH HENRY. 169 



series of observations upon ozone, concludes that its presence 

 in the air does not depend upon temperature but moisture. 

 He has observed traces of it when the thermometer was at 

 20° F. below and at 80° above zero. But in general it was 

 present in large quantities during the fall of rain and snow, 

 which may account for its greater prevalence near the sea- 

 shore than elsewhere. It appears to exist in great quantities 

 in dew, and to this fact has been attributed the remarkable 

 rusting effect produced on iron when exposed to this form 

 of precipitation of water. 



Malaria, or miasma. — In certain places, there is diffused 

 through the air an exceedingly minute quantity of a sub- 

 stance which has a powerful effect on the human system, 

 and frequently oifers in such districts a serious obstacle to 

 the cultivation of the soil. It is this which gives rise to in- 

 termittent fevers and perhaps to maladies of a more malig- 

 nant character. This substance is found in marshy and low 

 places where animal and vegetable matter of an aqueous 

 character is in a state of decomposition, but the winds which 

 pass over these places transport the malarious effluvia to a 

 distance and thus render whole tracts of country unhealthy. 



The corpuscules of this substance appear to adhere to the 

 molecules of water, and are elevated with the latter by the 

 ascending currents of air to heights which vary in dif- 

 ferent countries. Around the Pontine marshes, in Italy, 

 the malaria disappears at the height of from seven hun- 

 dred to one thousand feet, while in South America, accord- 

 ing to Humboldt, it is found at an elevation of three 

 thousand feet ; usually however its effects are exhibited with 

 intensity at a much lower elevation than that first mentioned. 

 It is also observed that humid air which transports miasma 

 is deprived of this noxious material in passing through 

 trees, and that in many cases, in the same neighborhood, a 

 screen of foliage is sufficient to produce a marked difference 

 between two places otherwise similarly situated. Double 

 screens of fine gauze also placed in the windows of sleeping 

 rooms answer a similar purpose, and should be resorted to in 

 all cases as a precaution wherever there is danger of disease 



