316 Intelligence mid MiscellaJieotis Articles. 



iron retards it. In the first case the metal has not lost its crystalline 

 structure ; in the second all crystallization is destroyed. Moreover, 

 the homogeneity of the surface must also diminish the influence of 

 laminated iron. 



It will be observed, that this opposition of effects is subordinate to 

 the part which the metal ought to play in the reaction ; for the zinc 

 onlj' dissolves, the iron serves merely as a negative pole. 



The briskness of the action is measured by the hydrogen disen- 

 gaged. This is abundant with ammonia, potash or soda, when la- 

 minated zinc and iron turnings are employed ; but, on the contrary, 

 it is more or less slow, according as the conditions are varied : 1 

 part of iron turnings, 3 parts of laminated zinc, and 300 cubic cen- 

 timetres of solution of ammonia, yielded crystals in ten days ; the 

 iron did not dissolve, its action was constant, and it was requisite 

 merely to replace the zinc from time to time as it disappeared. 



The crystals are deposited wherever there are inequalities. They 

 are very small if the action is rapid ; but when it is moderately slow, 

 they are very well-formed. They are always very brilliant when ob- 

 tained with ammonia ; when with potash or soda, they are entirely 

 opake : this happens because the greater part of the hydrate of zinc 

 is decomposed in these solutions, and a magma is deposited which 

 contains but few crystals. 



An experiment in which iron was replaced by lead, gave in four 

 mouths rather large crystals, considering the quantity of ammonia 

 employed. 



In another experiment commenced the same day, there were em- 

 ployed ammonia, copper turnings and laminated zinc ; after four 

 months there were deposited large prisms upon the inequalities of 

 the copper. It is to be observed, that during the whole of this time 

 the ammonia did not become blue, and yet it was left exposed to 

 the air. The author ascertained that in general the ammonia under 

 these circumstances is not rendered blue in the air, if the two metals 

 are completely immersed. 



This fact is so simple, that it is astonishing that it has not been long 

 known. Finally, it belongs to the series of observations which Davy 

 has made on the influence of saline solutions on copper. This metal, 

 in fact, cannot be attacked by oxygen, when in the presence of a 

 metal which is more electro-negative than it, in the medium under 

 consideration. 



This is so true, that if instead of zinc we make use of iron or tin, 

 or of metals in fact which are negative in ammonia with relation to 

 copper, the ammoniacal liquor becomes as blue in the air, at least as 

 rapidly as if copper alone were employed. 



The same does not occur if copjoer be immersed in a solution of 

 zincate of ammonia in contact with the air, but without metallic zinc. 

 The liquor, it is true, does not become coloured for some hours ; but 

 in twenty-four hours small crystals of hydrate of zinc are formed*. 



» If the arrangement be so made as not to immerse the whole of the 

 copper in order to favour the absorption of oxygen, tlie precipitation is much 

 more rapid and abundant ; the monoliydrate is deposited as a granular 

 powder, mixed with a few crystals. 



