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1825.] upon the same mixture ignited a second time, the proportion diminished; in a third operation it was sensible; in a fourth barely perceptible. The same mixture, however, by the addition of a new quantity of potash, again gained the power of producing ammonia in two or three successive operations; and when any mixture had ceased to give ammonia, the power was not restored by cooling it in contact with air.

Sir Humphry Davy refrains from drawing conclusions from these processes, observing with regard to the composition of nitrogen in these experiments, that till the weight of the substances concerned and produced in these operations are compared, no correct decision on the question can be made: I am anxious to be understood as imitating the caution of one whose judgment stands so high in chemical science; and therefore draw no positive conclusion from the experiment I have described, or from the results I have yet to mention. As, however, I think they may lead to elucidations of the question, I shall venture to give them, not with the minute detail of the preceding experiment, but in a more general manner.

Potash is not the only substance which produces this effect with the metals and vegetable substances. Soda produces it; so also does lime and baryta, the latter not being so effective as the former, or producing the phænomena so generally. The common metallic oxides, as those of manganese, copper, tin, lead, &c., do not act in this manner.

Water or its elements appear to be necessary to the experiment. Potash or soda in the state of hydrates generally contain the water necessary. Potash, dried as much as could be by heat, produced little or no ammonia with zinc; but re-dissolved in pure water and evaporated, more water being left in it than before, it was found to produce it as usual. Pure caustic lime, with very dry linen, produced scarcely a trace of ammonia, whilst the same portion of linen with hydrate of lime yielded it readily.

The metals when mixed with the potash appear to act by, or according to their power of absorbing oxygen. Potassium, iron, zinc, tin, lead, and arsenic evolve much ammonia, whilst spongy platina, silver, gold, &c., produce no effect of the kind. A small portion of fine clean iron wire dropped into potash melted at the bottom of a tube, caused the evolution of some