18 G

PURIFICATION.

and money have been squandered on endeavours to purify gas by means which werephysically impossible.

In a series of articles on the chemistry of coal-gas, which appeared in the second and thirdvolumes of the ‘Journal of Gas Lighting,’ the practical application of theoretical principlesto the purification of gas is explained with great ability, and with a perfect knowledge of thesubject, and we cannot do better than transfer to our pages the substance of the valuableinformation contained in those articles, so far as it refers to the present subject.

Taking impure coal-gas as it issues from the condenser, the impurities requiring removalwill be found to consist of ammonia, sulphuretted hydrogen, and carbonic acid, in propor-tions which, when common Newcastle coal has been employed for distillation, may be takenat 1^ part of ammonia, 8 parts of sulphuretted hydrogen, and 25 parts of carbonic acid inevery 1000 measures of the gas. From this it appears that the quantity of ammonia presentis not nearly equal to the saturation of the sulphuretted hydrogen, and still less, therefore, tothat of the carbonic acid also. Hence it is found that neutral metallic salts, such as thesulphate of iron, do not remove the whole of the sulphuretted hydrogen; nor neutral earthysalts, such as the muriate of lime, the whole of the carbonic acid. One great cause of thedifficulty of purification, therefore, arises from the deficiency of ammonia, and not from itsexcess; for that necessitates the employment of some other alkali or alkaline earth, of whichlime is the cheapest.

Common lime, used in a dry-lime purifier, is the most effectual, the most economical, andthe least hurtful agent by which the impurities in coal-gas can be removed; and next to itmay be classed lime used in a wet purifier, though the latter is much less effective, and isapt to leave carbonic acid in the gas. Of course this goes no further than to the absorptionof the acidulous compounds contained in the gas, such as carbonic acid and sulphurettedhydrogen. But something may be learned, nevertheless, from this peculiar fitness of limeeven in respect to the removal of the alkaline compounds. Neither carbonate of lime northe hydro-sulphate of lime possess any affinity for those naphtha vapours or hydrocarbonson which the luminosity of gas depends. Hence the lime, when fully saturated with car-bonic acid and sulphuretted hydrogen, absorbs none of the light-giving principles of coal-gas, which pass on untouched. A very different result, however, takes place with the oxides,carbonates, and sulphurets of the metals.

Though the ammonia cannot be removed by hydrated lime, the salts of that earth possessthe property of absorbing it from the gas; and as lime causes the least absorption of illu-minating matter, it follows that the salts, of lime are on that account better adapted than anyother substance for the removal of the alkaline or ammoniacal impurities from coal-gas. Itmay, indeed, be taken as a generally admitted fact, that when any of the salts of lime, suchas the nitrate, sulphate, or muriate, are so placed that the current of impure gas may firstpass through or upon them, and afterwards traverse the filled compartments of a dry-limepurifier, then the greatest known perfection of coal-gas purification is obtained. Of thesesalts the dry sulphate of lime is the only one not included in any valid patent in this country,though it constitutes the basis of a very important patented invention much used in France,the result being, under favourable circumstances, a valuable manure.