258 EIGHTH REPORT — 1838. 



observations of Marcet, Scoresby, and others, show that the water 

 of the ocean is rather denser, or contains more saline matter in 

 torrid and temperate zones than in high latitudes, yet from 3^ 

 to 4 per cent, of solid salts may be taken as the general average. 

 And as these have a complex constitution, so the results of their 

 action on iron and water containing air are complex; and expe- 

 riments are yet wanting to enable a perfect rationale of the pro- 

 cess to be given and its results precisely stated. 



15. In the first instance the actions already described, in the 

 case of air and pure water at low temperatures, take place and 

 give rise to the oxides of iron ; and as the sea water almost 

 always contains carbonic acid, a portion of these is resolved into 

 carbonate of iron. 



16. As in pure so in sea water, when iron is deeply immersed 

 the oxide produced is the magnetic in the first instance, the 

 iron becomes covered with a light buff coat of rust; but if the 

 vessel be shallow, the sesqui-oxide is formed gradually from it. 

 In each case, the first appearance of action which the fluid pre- 

 sents is the formation of numerous slight green streaks in it. 

 These form usually in about thirty minutes from immersion of 

 the metal, and appear to be protoxide in progress of transition 

 into magnetic oxide and sesqui-oxide, of which latter oxides a 

 large precipitate soon forms at the bottom of the vessel. 



17. But as sea water likewise contains chlorides of sodium 

 and magnesium, the carbonate of iron is, in part at least, de- 

 composed, and a subchloride of iron is formed which unites 

 with a part of the sesqui-oxide of ii'on, having previously as- 

 sumed the slate of sesquichloride, and forms with it an inso- 

 luble compound. 



18. It happens hence, after a mass of iron has lain for a con- 

 siderable time in a limited quantity of sea water, that the latter 

 holds carbonate of soda in solution, and the further action be- 

 comes very slow, and that a hydrated carbonate of magnesia 

 has deposited on the iron. 



19. It would appear also, that the sulphates are in part de- 

 composed, the sulphuric acid passing to the iron and forming 

 a basic insoluble sulphate, and the lime an insoluble carbonate, 

 with the carbonic acid of the water. But sulphiu'ic acid is by 

 no means uniformly to be detected in the ochreous deposit 

 formed by the action of sea water on iron, nor indeed chlorine 

 either. But besides chlorides, sea water contains bromides and 

 iodides, and of the part which these play in the decompositions 

 conseqvient on the action of iron, it must be confessed we are as 

 yet wholly ignorant. Analogy, however, gives reason to pre- 

 sume they play similar parts to the chlorides. 



