CAUSES OF FEVER. 
165 
progress upwards on the 20th August, and on the 
3rd September, the first case of fever occurred on 
board H.M.S.V. ‘ Soudan,’ exactly the fifteenth day in 
both ; but when Lieutenant Webb ascended the river 
in July, 1842, the first sickness evidenced itself on 
the sixteenth day. 
Of the predisposing causes arising out of the con- 
dition of the atmosphere, it would be impossible to 
speak with any certainty, since the most delicate chemi- 
cal tests have failed to elicit the presence of any of the 
deleterious gases supposed to exist in those regions. 
From the date of entering the river, we examined 
both air and water several times during each day and 
night with the greatest care, and cmdd not detect 
sulphuretted hydrogen in either, and only a slight 
trace of carbonic acid gas in the air. Still there can 
be no doubt there obtains at some seasons — especially 
about the conclusion of the rainy and beginning of the 
dry period — a certain pecidiarity of atmosphere, — 
call it miasm, malaria, or any other name, — which, 
though inappreciable by chemical agency, operates 
most powerfully on Europeans. Even on those of the 
most robust frame, and those who escaped the river 
fever, the climate gave rise to an indescribable languor 
and want of nervous energy, under which the strongest 
constitution must have yielded. 
AVe can speak with greater confidence of the effects 
of solar heat as acting in three ways ; first, in evolving, 
after the rains, those emanations from the soil, which 
