ORIGIN OP THE TROPICAL MALARIA. 27 



The sulphuretted hydrogen, proved by Mr. Daniell to 

 be largely generated on the African coast, is an amply 

 sufficient cause of the destruction of the copper, and no 

 future contractor need fear the result of a similar trial. 

 The origin of the gas is, with much probability, traced 

 to the reaction between the masses of decaying vege- 

 table matter brought down by the tropical rivers, and 

 the sulphates contained in sea-water. The unhealthi- 

 ness of mangrove swamps, and of low districts in the 

 vicinity of the sea in various parts of the world, but 

 especially within the tropics, appears also to be owing 

 to the formation of sulphuretted hydrogen, where the 

 decaying leaves come in contact, in large masses, with 

 sea-water. This is the case in all mangrove thickets, 

 which always border on the sea, and often extend to a 

 short distance from shore. The insalubrity of the 

 west coast of Africa, as a whole, is but too well estab- 

 lished. It is known that the existence of only 15 1 00 th 

 part of sulphuretted hydrogen in the air acts as a 

 direct poison on small animals. In large quantities it 

 is rapidly destructive of human life, and, in smaller 

 doses, its disgusting stench causes excessive depression 

 and languor, as well as nausea ; and there can be little 

 doubt that the tropical malaria, which causes these ef- 

 fects, owes its virulence to the presence of this gas. The 

 formation of sulphuretted hydrogen in sea-water, by the 

 action of decaying vegetable matter, was proved by Mr. 

 Daniell, who found that while leaves allowed to remain 

 for a considerable time in a solution of muriate of soda 

 remained perfectly fresh and untainted, similar leaves, 

 placed in a jar containing sulphate of soda and water, 



