322 Proceedings of the British Association. 



magnesia, when from the slowness of their decomposition they 

 continue sterile ; and proposed in such cases to accelerate the dis- 

 integration by pouring upon the sub-soil dilute sulphuric acid. 



Mr. Prideaux communicated the results of inquiries into the 

 causes of the increased destructihility of modern copper sheathing. 

 Experiments made on various kinds of copper sheathing immers- 

 ed in sea water, showed, that in the laboratory, under parallel 

 circumstances, they do not observe the same order of durability 

 and waste, as they had done in use. The cause of comparative 

 waste appears therefore to be, in part at least, due to external con- 

 ditions ; and of these, two classes may be noticed ; one depending 

 on the connexion with the ship, the other on the circumstances 

 of her employment. Of the first class, two suggested them- 

 selves, — the position on the ship's side, and the nails by which the 

 copper is fastened. The lower part of a ship's copper seems to 

 suffer much less than the upper, so long as she continues in deep 

 water ; but when she grounds at low water, if on black mud, 

 this part suffers most from the action of sulphuretted hydrogen, 

 peeling off in blue flakes. The influence of the nails oflers rather 

 more chemical interest. They are never of pure copper, and 

 being very numerous, all in contact with the copper sheets, whilst 

 their heads present also a considerable metallic surface to the salS 

 water, they may produce very decided effects, either preservative 

 or destructive, by a slight electro-chemical difierence. Various 

 experiments were tried, which proved that most nails are destruc- 

 tive. The conclusion resulting was this, that the nails might be 

 rendered slightly electro-positive to rolled copper, by the addition 

 of zinc, whicii would not injure their flexibiUty nor enhance their 

 cost. The test by the galvanometer would, after a little prac- 

 tice, be easily applied, in making up the metal for casting them. 



Another mode of protection is offered, by coating the copper 

 when new, with fish oil, which in one instance has been of sig- 

 nal service. The preservative effect of coal-tar was also noticed. 

 This tar had trickled down over the copper from the wood- work 

 above, and had crossed the sheets just where most subject to the 

 wash and friction ; and whilst the naked metal had been quite 

 worn away, the coal-tarred streaks remained entire ; the surface 

 of the copper, on melting off the tar, being as perfect as when 

 fresh from the roll. But it remains to be seen whether it will 

 keep a clean surface free from organic adhesions and earthy incrus- 



