38 ON THE ALLOYS OF COPPER AND ZINC. 
conditions depending, as he thinks, upon the too great proportion of zinc which 
is used in the preparation of the alloy, yellow-metal, as well as to changes of 
composition produced by hot rolling. 
I cannot agree with this conclusion. In my own opinion, the other alterna- 
tive which Bobierre has suggested, namely, peculiar arrangement of the molecules 
of which the alloy is composed, furnishes the true explanation of the difficulty. 
It is, however, possible, that at times, when the temperature of the reverberatory, 
in which the alloy is heated before passing to the rollers, is not properly regulated, 
zinc may be burned off from the exterior portions of the sheet, and that the alloy 
richer in copper which would thus be formed may subsequently be pressed into the 
body of the sheet during the operation of rolling. An alloy destitute of homogeneity 
would result from this treatment which could hardly be durable in any event. 
Instances of this sort must nevertheless be rare, for no part of the process is watched 
by the manufacturers more scrupulously than this. 
It must also be borne in mind, that, of the enormous quantity of yellow-metal 
which is now used by the merchant-vessels of Great Britain and of this country, — 
all of which is composed of sixty parts of copper and forty parts of zinc, and rolled 
hot, — only a comparatively small portion passes into the friable condition to which 
I have alluded. 
In most cases the alteration which the sheathing undergoes is gradual and regular, 
and the portion which remains after the wear of several years is still malleable. Indeed, 
the absolute amount of sheathing which becomes friable is entirely out of proportion 
with the annoyance to which it subjects ship-owners; * for it rarely happens, even 
in the worst instances, that more than one third of the sheets upon a vessel become 
friable, the remainder being in good condition. 
The friability is therefore a purely accidental occurrence, and by no means a 
necessary consequence either of hot rolling or of the presence of 40 per cent of 
zinc in the alloy, as has been implied by Bobierre. Iam strongly of opinion, that 
it might be in every instance entirely obviated by methodically annealing or tempering 
* Whenever the destruction of a portion of the sheathing requires that a ship be hauled up for repairs, 
it is customary, since this operation is an expensive one, to resheathe the vessel entirely; for as the friable 
sheets are interspersed among the others, and since all the sheets are somewhat worn, it would be bad 
economy to attempt any partial repairs. Of course the owner of the vessel regards the entire suit of 
sheathing as being worn out, and forms his opinion of the durability of yellow-metal in accordance with 
this view. 
f Thises p. 77. —— 
