260 REPORT—1840, 
the lapse of some days, the alloys (2 Zn+Pb), (Zn+Pb), and 
(Zn+2 Pb) have reduced a few scattered crystals of lead; but 
the remaining alloys, (Zn+3 Pb) and (Zn+4 Pb), act in all re- 
spects precisely as the lead itself towards its own salts. 
247. When a similar set of alloys are placed in a solution of 
nitrate of copper, a metal which is reduced from its salts both 
by zine and lead, then the zinc and the alloys (4 Zn+ Pb) and 
(3 Zn+ Pb) reduce the nitrate to metal, and the lead does so 
likewise. The alloys (2 Zn+Pb), (Zn+Pb), and (Zn+2 Pb) 
reduce the salt to deutoxide and metal mixed; but the alloys 
(Zn +3 Pb) and (Zn +4 Pb) reduce the nitrate to deutoxide alone, 
without reduction of metal. From the relations in affinity for 
oxygen between copper, zinc and lead, it was to be presumed, 
that all the alloys of the two latter metals would reduce copper; 
but it is remarkable that all the alloys between (2 Zn+ Pb) and 
(Zn+4 Pb) have less power of reduction than lead alone, while 
the alloys (4 Zn+ Pb) and (3 Zn + Pb) have at least equal power 
with zinc alone. 
Analogous phenomena occur when solutions of other metals, 
reducible by either zinc or lead, are used; and also when other 
metals, as the alloys of copper and zinc, or copper and tin, are em- 
ployed, so that no prediction can be made, from the known affini- 
ties of the component metals towards a saline solution, what shall 
be the affinities towards the same solution of their atomic alloys. 
248. In this class of reactions it by no means always happens, 
that both metals of the alloy, although both separately soluble in 
the electro-negative element of the saline solution experimented 
on, are dissolved in the ratio in which they exist in the alloy ; 
nor is it always the most electro-positive metal of the two of 
which the largest amount is dissolved. The presence of each 
metal, and of its oxides, affects the affinities of the other of them, 
instances of which we have in the alloy of silver and platina, 
soluble in nitric acid, &c. As, however, the treatment of the 
general question of the action of alloys, when immersed in acid 
or saline menstrua on the solvent, and on each constituent metal, 
does not properly belong to the present subject, and a sufficient 
general indication of their bearing upon it has been given, I re- 
serve the details for another occasion, and pass on to remark the 
practical uses to the engineer of Tables IX. and X. 
249. In Table IX. it will be seen that the twelfth column gives 
the amount of loss per square inch of surface of cast iron, with 
the skin removed by turning or planing (during a period com- 
parable with all the preceding experiments), in contact with 
brass, and various analogous alloys of zinc and copper, and also 
with copper and with zinc singly. Thus the engineer is enabled 
