300 MR. JOHN LEIGH ON THE 



originated with the late Dr. Henry, and has of late been 

 much recommended by Dr. Fyfe. The method consists 

 in mixing chlorine with the gas, allowing the mixture to re- 

 main for some time in the dark, and then observing the 

 diminution of bulk, and estimating half the diminution as 

 olefiant gas. Chlorine has the property of combining with 

 and condensing the olefiant gas and hydro-carbon contained 

 in coal gas, whilst in the dark it exercises no action on the 

 light carburetted hydrogen and other constituents. This 

 is an attempt to estimate the value of a gas by the mere 

 amount of olefiant gas that it contains ; but the method of 

 accomplishing this is liable to constant errors. Formerly 

 the whole of the diminution of volume was observed, and 

 one-half of the entire amount estimated as olefiant gas. It 

 was overlooked that a portion of the chlorine itself under- 

 went absorption, and increased the apparent diminution, 

 thus giving too large a volume of olefiant gas. Dr. Fyfe 

 detected this, and proposed, therefore, to observe first, how 

 much chlorine was absorbed alone by the confining water, 

 and then to deduct this amount, from the total absorption, 

 dividing the remainder as before. But this method is ex- 

 tremely fallacious, and, though still practised and strongly 

 recommended by Dr. Fyfe, is utterly untrustworthy. Dr. 

 Fyfe estimates the absorption of chlorine alone at 1 or 2 

 per cent, within the time employed for examination. I 

 have found it to vary from 2 to G per cent. The rate of 

 absorption varies with the diameter of the tube employed. 

 But what is of more consequence, and completely vitiates 

 Dr. Fyfe's results, or any results obtained by this process, 

 is the fact, that the rate of absorption of chlorine varies with 

 the dilution of the latter by any other gas. The rate of ab- 

 sorption, when mixed with atmospheric air or any other gas, 

 is not the same as that with pure chlorine alone. Dr. Fyfe 

 has either not tried this, or has entirely overlooked it; and it 

 is as great an oversight as that which he has sought to cor- 



