In all probability such limit may be
reasonably taken at 0.1 milligramme of either sulphur or phosphorus
(calculated as elementary bodies) per 1 litre of acetylene, _i.e._,
0.0-1.1 grain per cubic foot; a quantity which happens to correspond
almost exactly with a percentage by weight of 0.01. Owing to the atomic
weights of these substances, and the very small quantities being
considered, the same limit hardly differs from that of 0.01 per cent. by
weight of sulphuretted hydrogen or of phosphine--it being always
recollected that the sulphur and phosphorus do not necessarily exist in
the gas as simple hydrides. Keppeler, however, has suggested the higher
figure of 0.15 milligramme of either sulphur or phosphorus per litre of
acetylene (=0.066 grain per cubic foot) for the maximum amount of these
impurities permissible in purified acetylene. He adopts this standard on
the basis of the results of observations of the amounts of sulphur and
phosphorus present in the gas issuing from a purifier charged with
heratol at the moment when the last layer of the heratol is beginning to
change colour. No limit has been given for the removal of the ammonia,
partly because that impurity can more easily, and without concomitant
disadvantage, be extracted entirely; and partly because it is usually
removed in the washer and not in the true chemical purifier.
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