344 Analysis of Scientific Books. 



In his last paper, the President has referred to the circum- 

 stance of the carbonate of lime and magnesia forming upon sheets 

 of copper, protected by a quantity of iron above T £ parts, when 

 these sheets were in harbour and at rest. 



" The various experiments," says Sir Humphry, " that I have 

 caused to be made at Portsmouth, shew all the circumstances of 

 this kind of action, and I have likewise elucidated them by experi- 

 ments made on a smaller scale, and in limited quantities of water. 

 It appears from these experiments, that sheets of copper at rest in 

 sea-water, always increase in weight from the deposition of the 

 alkaline and earthy substances, when defended by a quantity of 

 cast-iron under i: -!- ff of their surface, and if in a limited or con- 

 fined quantity of water, when the proportion of the defending 

 metal is under IS ^. With quantities below these respectively 

 proportional for the sea, and limited quantities of water, the copper 

 corrodes ; at first it slightly increases in weight, and then slowly 

 loses weight. Thus a sheet of copper 4 feet long, 14 inches wide, 

 and weighing 9 lbs. 6 oz., protected by T J 5 of its surface of cast- 

 iron, gained, in ten weeks and five days, 12 drachms, and was 

 coated over with carbonate of lime and magnesia: a sheet of 

 copper of the same size protected by t Jq, gained only 1 drachm 

 in the same time, and a part of it was green from the adhering 

 salts of copper ; whilst an unprotected sheet of the same class, 

 both as to size and weight, and exposed for the same time, and as 

 nearly as possible under the same circumstances, had lost 14 

 drachms ; but experiments of this kind, though they agree when 

 carried on under precisely similar circumstances, must of necessity 

 be very irregular in their results, when made in different seas and 

 situations, being influenced by the degree ot saltness, and the 

 nature of the impregnations of the water, the strength of tide and 

 of the waves, the temperature, fyc. 



" In examining sheets which had been defended by small quan- 

 tities of iron in proportions under ^^ and above t^^, whether 

 they were exposed alone, or on the sides of boats, there seemed to 

 me no adhesions of confervas, except in cases where the oxide of 

 iron covered the copper immediately round the protectors ; and 

 even in these instances such adhesions were extremely trifling, 

 and might be considered rather as the vegetations caught by 

 the rough surface of the oxide of iron, than as actually growing 

 upon it. 



" Till the month of July, 1 824, all the experiments had been tried 

 in harbour, and in comparatively still water ; and though it could 

 hardly be doubted, that the same principles would prevail in cases 

 where ships were in motion, and on the ocean, yet still it was 

 desirable to determine this by direct experiment ; and I took the 

 opportunity of an expedition intended to ascertain some points of 

 longitude in the north seas, and which afforded me the use of a steam- 



