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ON THE ACTION OF AIR AND WATER UPON IRON. 263 
- collection of strictly atomic alloys of the two practically impor- 
tant classes of brass and oe, has not been made hereto- 
fore. 
The principal experiments on the properties of this class of 
alloys published, are those of Margraff; but his were not 
atomic alloys, nor made in a way likely to ensure a knowledge 
of their constitution. I have therefore deemed it worth while 
to make some experiments on the properties of these alloys, and 
have given the results in Tables XIV. and XV.: as these nearly 
explain themselves, it is necessary to make but few remarks on 
them. The alloys of zinc and copper were all made in close 
vessels; the copper was fused first, the zinc in another part of 
the same bent wrought iron tube, coated and lined with porce- 
lain clay and plumbago. The zinc was gradually brought in 
contact with the copper: the apparatus excluded air, and was 
continually agitated, until the alloy was poured into a mould of 
cast iron, in which it was cast into a long strip, which solidified 
instantly. About seven pounds weight of each alloy were formed 
at once, and the constitution of each, where any cause of doubt 
existed, was verified afterwards by an assay. Their composition 
therefore is rigidly assigned—a circumstance which it is con- 
ceived gives their properties, so far as they have been ascer- 
tained, more than usual value. 
255, The modulus of cohesion given is higher considerably 
than those found by Sir John Rennie for copper and brass 
(2 Zn+Cu), or commonly assigned to zinc. I have no doubt, 
however, of the present being correct, and the difference arises 
probably from the superior purity of the metals used by me. 
256. The immediate change by the addition of only one 
eighth of an atom of zinc to the alloy (2 Zn+Cu), from a tough 
yellow alloy to a white one of extreme brittleness, is very re- 
-markable. The alloy (5 Zn+Cu), and all the alloys of copper 
and zine having more constituent zinc than (17 Zn+8Cu) are 
electro-positive to cast iron, or protect it in solvents ; yet when 
_ the alloy of copper is reduced to the ratio of (25 Zn+Cu), or 
(100 Zn + Cu), the compound becomes again electro-negative to 
cast iron*. These indicate, in a forcible manner, that these 
_ latter are not simple alloys, but mixtures. It may be added, 
that the reduction to the law of volta-equivalents of the losses 
in the seventh column, may enable us to discover what is the 
constitutional arrangement of the alloys themselves. 
257. It should be remarked, before leaving the subject of 
_ these alloys of Zn+ Cu, that their specific gravities, as experi- 
* § 243. 
