1820.] Dr. Henry's Experiments on the Gas from Coal. 35 



conveyance. Having, I hope, furnished documents for solving^ 

 questions of this sort, I shall proceed to describe in what mau~ 

 uer the facts were ascertained. 



Method of Aiialysis. 



1. Determination of the Proportions of Carbonic Acid and' 

 Sulphuretted Hydrogen Gases in Coal Gas. — In experiments 

 formerly made on this subject, I employed the agency of chlorine 

 to condense both these impurities, and estimated how much oP 

 the absorption was due to each, by a rule which I have stated.* 

 Recent experience, however, has led me to distrust this method; 

 and after comparing the effects of several other agents, by expe- 

 riments on mixtures of known composition, I now prefer the 

 white carbonate of lead, precipitated Irom acetate of lead by car- 

 bonate of ammonia without heat, and, therefore, fully saturated 

 with carbonic acid. This precipitate it is better not to dry, but, 

 after washing it sufficiently, to leave it under as much water as 

 will give it, when wanted for use, a due degree of fluidity. This. 

 mixture may be applied by means of a tube of the capacity of a 

 cubic inch, divided into 100 equal parts, and accurately ground 

 into a short and wider piece of tube, which ought not to contaia' 

 more than three or four-tenths of that quantity. The wider tube 

 being filled with the fluid carbonate of lead, and placed with its, 

 mouth upwards under water, the graduated measure full of gas 

 is fitted to it ; and the gas and liquid are brought into contact 

 by alternately inverting the two tubes, all violent agitation being 

 carefully avoided. The sulphuretted hydrogen is thus absorbed,.- 

 and the carbonic acid, being left untouched, is afterwards taken 

 out from tlie same portion of gas by a similar use of solution of 

 l>ure potash. 



2. To ascertain the Proportion of Olefiant Gas in the Residue 

 left by Potash. — From 25 to 30 hundredths of a cubic inch of 

 chlorine gas are passed into a tube of the diameter of about 

 -i^ij^ths of an inch, accurately divided into hundredths of a cubic 

 inch ; and the volume of the chlorine is noted when actually in 

 the tube, to avoid errors from its absorption in rising through the 

 water. To this is admitted half a cubic inch (equivalent to 50 

 measures) of the gas under examination, and the mixture is left, 

 <;xcluded from the direct hghtof the sun, and perfectly quiescent, 

 for 15 minutes. At the expiration of this time, the remainder is 

 noted, and the diminution which has taken place being divided 

 by 2, the quotient shows the quantity of olefiant gas in 50 mea- 

 sures of the mixture. This process, 1 am aware, however, does 

 not give results of perfect accuracy ; for, in addition to other 

 sources of fallacy, 1 find that chlorine begins to act on carbu- 

 retted hydrogen much sooner than is generally supposed,t though 



♦ Piiil. Trans. 1808, p. 295. 



f \Ylii)c this sheet was passing through the press, I have noticed a passage ia 



c2 



