in the Waters of the Ocean. 1 1 



only be found necessary to use protection in sulphuretted 

 waters, and that the action of the chlorides alone might not 

 be more than sufficient to preserve the copper from deposits. 



Another motive for this change may be found in an observa- 

 tion of Sir H. Davy, viz. that a "common cause of the adhe- 

 sion of weeds or shell-fish is the oxide of iron formed and 

 deposited round the protector. In the only experiment in 

 which zinc has been employed for this purpose in actual ser- 

 vice, the ship returned, after two voyages to the West Indies 

 and one to Quebec, perfectly clean. The rudder, which was 

 not protected, was corroded in the usual way*." 



But it is impossible not to speculate upon the origin of the 

 deleterious gas which has thus been found to contaminate the 

 sea upon the western coast of Africa, in such enormous quan- 

 tities through an extent of more than sixteen degrees of lati- 

 tude, and reaching in places forty miles seawards, making 

 altogether an area of 40,000 square miles in extent. 



Volcanic action seems naturally to suggest itself, but is 

 negatived by the absence of any other indications of such ac- 

 tion along this line of coast; and I think that I shall be able 

 to convince you, by the evidence of experiment, that the real 

 cause may be found in the mutual reaction of the immense 

 quantities of vegetable matters, which must be brought down 

 by the intertropical rivers, and the sulphates of the sea 

 water. 



The idea was suggested to me by a memoir of my friend 

 Dr. Malcolmson, in the Geological Transactions, who specu- 

 lates upon the origin of sulphuretted hydrogen in the saline 

 lakes of different parts of the world, being •* the decomposi- 

 tion of the sulphates in the water by the carbonaceous matter 

 of vegetables." I tested this hypothesis by experiments in 

 the following way : — 



On the 2nd of November, 184-0, I placed a quantity of 

 newly-fallen leaves in three glass jars capable of holding about 

 a gallon and a half of water. 



No. 1. Upon the first I poured about a gallon of New 

 River water. 



No. 2. Upon the second I poured about the same quantity 

 of the same water, in which three ounces of common salt had 

 been dissolved. 



No. 3. Upon the third, the same quantity of water, in which 

 three ounces of crystallized sulphate of soda had been dis- 

 solved. 



The three jars were then placed in a warm chamber, the 



* Phil. Trans. 1826, p. 420; [or Phil. Mag. Second Series, vol. i., p. 198.] 



