TRANSACTIONS OF THE SECTIONS. 43 



CHEMISTRY. 



Some Inquiries into the Causes of the increased Destructihility of Modern 

 Copper Sheathing. By J. Prideaux, F.G.S. 



On the 29th of May last year, the author had been requested by Mr. Owen, of H.M. 

 Dockyard, to analyse some sheet copper from the sheathing of the Sanspareil, which 

 was still in good condition after thirty years' wear. The sample gave about 0'25 per 

 cent, of alloy, chiefly tin, zinc and iron (see Table I.). This contrasted with an- 

 other sample, quickly worn out, and which had given no quantity of alloy sufficient 

 to weigh. These instances coincided with two recorded analyses of Sir H. Davy 

 and Mr. R. Phillips, the former having detected in a sample of very durable sheath- 

 ing about 1 •400th of tin ; the latter having found one which had lasted hardly four 

 years, the purest copper he had ever analysed ; and further, with the reputed in- 

 feriority of the recently prepared sheathing of the Royal Navy, which must have 

 been much purified by repeated fusions. 



The inference was, that the presence of tin and zinc was favourable to the dura- 

 bility of the copper ; not perhaps from any advantage in the alloy, but as a guarantee 

 of the absence of suboxide, which melted copper was liable to dissolve in refining, 

 but which could hardly be supposed to coexist with zinc and tin : and nothing 

 could be more injurious to the durability of the copper than being thus penetrated 

 with oxygen, which must facilitate the action of the sea water, mechanically as well 

 as chemically. 



To bring this inference to the test of direct comparison, four other samples were 

 submitted to analysis. 



1. From the Minden, worn 17 years, annual loss 0'45 per cent. 



2. „ Plover, „ 5 „ „ 11 „ 



3. „ Linnet, rapidly destroyed, no sheet left sound enough to weigh. 



4. New sheathing from the Portsmouth mills. 

 The following table gives the results : — 



Table I. 

 Plover. Linnet. 



Copper, worn 5 yrs 



Loss per cent, per annum 11 unknown 



Specific gravity 8-95 8-97 



Percentage of other metals. 



. fTin 0-1 0-07 



I Zinc 0-21 0-15 



^■i Iron 013 36* 



1= I Silver 009 006 



(.Lead trace. 



perishable. durable. 



It will be observed that two of these samples wore remarkably well, and the 

 other two suffered rapidly ; but that there is no such distinction in the proportions 

 of tin and zinc as to confirm the previous inference, or indeed to explain the dif- 

 ferences of durability from the composition of the metal. 



The cause of waste, then, not appearing in the analyses, was sought in the phy- 

 sical qualities of the samples ; but neither their comparative hardness, tenacity, 

 grain in fracture, nor colour, presented more consistent relations to their wear. The 

 specific gravity only coincided with the durability, the heaviest having worn the 

 best. The difference is small ; but from these and other considerations, Mr. Prideaux 

 had been induced to recommend that sheathing copper should be finished by cold 

 rolling, to increase the pressure, and harden the metal, the better to resist friction. 



It was another question, how far the waste was due to any defects in the sheath- 

 ing itself, or to external causes. To determine this, a slip from each sample, of 

 equal surfaces (4 X 0*5 inch), were immersed each in a pint of sea-water, the five 

 vessels being placed each side by side, so as to put them all in like conditions. Sea- 



* These two quantities of iron were probably a little increased by dust from the fire in 

 evaporation. 



