Abstract
The preparation of tree radicals from lead tetraethyl and tetramethyl has been described by Paneth and Hofeditz* and, having fully confirmed their work with the former compounds we sought, at the commencement of this investigation, to apply a simi1ar technique to an analogous problem : the preparation of atomic hydrogen by the thermal dissociation of metallic hydrides. preliminary experiments on the dissociation of certain of the more accessible hydrides showed, however, that those of sulphur, selenium, tellurium, arsenic, antimony and tin, under the conditions employed, failed to yield hydrogen which was capable of removing even the most sensitive metallic mirror—such a mirror, we had been able to show, disappears quickly and completely when atomic hydrogen is present. It is really not surprising that the hydrogen from these compounds is inactive since the accepted accommodation coefficients of the elements involved are relatively low. In this respect, the hydrides of lead and bismuth promised greater success. Hitherto, the hydrides of lead and bismuth have been obtained only in minute quantities, and have been characterized entirely by circumstantial evidence, hence it was necessary to discover means whereby they might be prepared on a larger scale. Bismuth hydride, as it proved, did not present much difficulty ; the yields from the dissolution of a magnesium-bismuth alloy in acid, although email, sufficed for our purposes. On the other hand, the results accruing from the whole of our work on the lead compound are so meagre, and so dependent on inexplicable circumstances, as to leave a sense of uncertainty regarding the nature of the traces of lead-carrying body produced in the operations, Neither the dissolution of the magnesium alloy, nor the reduction of lead salts by magnesium gave the hydride, whilst a change of the alloying metal to lithium, the use of which in these preparations is new, resulted in the merest trace of a very impure, volatile lead compound from 20 gm. of the alloy, and was without promise of yielding either a pure product or a sufficient quantity. About fifty experiments with the oscillating are, sometimes in its original form* and sometimes, modified so as to reduce the by products contaminating the lead-containing condensate, agreed down to the minutest detail with the recorded observations of Paneth and Norring, but, incidentally, gave uselessly small fields, they found the presence of carbon to be a condition essential to the appearance of lead in the effluent gases, and of this there can be no question. That our lithium contained carbon, whereas our magnesium was practically free from it is also significant in this connection.