1070 Experiments and Papers, [No. J 44. 



the zinc, without giving greater protection. The practical inference is, 

 that the protectors should be massive, but of small surface, and placed 

 where they can be conveniently renewed ; further, that it is an error to 

 suppose, that merely zincing the heads of nails or rivets would suffice ; 

 such a method would be worse than useless, for the quantity of zinc 

 thus used, would be insufficient to sustain the protection, and the corro- 

 sion which follows always commences at the points of contact with the 

 late protector, thus endangering a vessel in its most important parts. 



The zinc undergoes oxydation. If moving through sea water the oxyde 

 is washed off and dissolved ; — if quiescent, the oxyde is deposited in a 

 spongy white coating on the iron. Besides oxydation the zinc also suffers 

 considerable loss by its structure becoming granular and brittle, and 

 readily disintegrated by friction. 



11th. I have tried many other metals beside zinc, and found only 

 one superior to it in protecting power. This is cadmium, which 

 presents advantages of extraordinary value, were these not counter- 

 balanced by the scarcity and dearness of the metal. While cadmium 

 protects the iron with equal energy, its own corrosion is little more 

 than one-third that of the zinc, and instead of becoming granular and 

 incoherent like the zinc, its surface is always bright and solid. The 

 present price of cadmium of course forbids its employment ; but the 

 high price I believe is attributable to there having been hitherto no use 

 discovered for the article, which exists to the amount of 3 per 100 in 

 many of the zinc ores of Great Britain. Were cadmium cheaply obtain- 

 able, it would at once be resorted to as an iron protector ; its influence 

 on copper I have not yet studied. 



Protection of Tin Plate. 



12th. I have extended this enquiry to the protection of tin plate, and 

 with rather interesting results. Tin plate is perfectly protected by pro- 

 portionally much smaller quantities of zinc than are required for iron. 

 The surface of the tin fouls very slowly, the fouling is caused by the 

 diffusion over it of the oxide of zinc. This is readily removed by 

 washing it, by a brush or coir scrubber, with a weak alkaline liquid 

 (a solution of one lb of saji-mati in 4 pounds of water is sufficient). 

 This fact I represent as one of considerable value, and calculated to lead 

 to much economy in the construction of buoys for Salt-water harbours, 



