36 Dr. Henry's Experiments on the Gas from Coal. [Jan, 



within the pericxl mentioned, and in such narrow tubes, it does 

 not occasion a sensible diminution of bulk. The method 

 described may, therefore, be considered as affording a tolerably 

 near approximation to the proportion of olefiant gas ; and as all 

 the varieties of coal gas were subjected to the test under pre- 

 cisely the same circumstances, the errors must have been of 

 nearly the same amount in all cases, and cannot materially inter- 

 fere with the fair comparison of the different specimens of coal 

 gas, so far as respects their proportion of olefiant gas. 



3. To ascertain the Qiiaiititi/ of Combustible Matter in gas 

 which had been deprived only of sulphuretted hydrcJgen and car- 

 bonic acid, a mixture of the gas with a due proportion of oxygen 

 gas was fired by the electric spark over mercury. This method 

 I preferred to slow combustion, carried on with the apparatus 

 which I have described in the Philosophical Transactions for 

 1808, solely because, when a great number of experiments are 

 necessary, as in this inquiry, the method of detonation is 

 attended with a great saving of time. But on all occasions 

 where only few experiments are required on gases of great com- 

 bustibility, I pi'efer slow combustion, both on account of greater 

 safety to the apparatus, and, from the quantities that may be 

 consumed, of greater accuracy also. When rapid combustion is 

 practised, I believe that, on the whole, more accurate results are 

 gained by firing the gas at one operation properly conducted, 

 than at two. The latter method seems to have been preferred by 

 M. Berthollet ; but so far as my experience goes, it is more apt 

 to precipitate charcoal from the gas. 



To burn each measure of the early and more combustible pro- 

 ducts of gas, I employed from three to four measures or upwards 

 of oxygen gas, the degree of purity of which had been ascer- 

 tained. The volume being noted after firing, and again after" 

 agitating the residue with liquid potash, the last diminution, 

 ishowed the quantity of carbonic acid. The gas left by potash 

 was next analyzed by combustion with a due, proportion of pure 

 liydrogen,* which showed how much of the residue was oxygen, 

 aad how much azotic gas. If more azote was found than had 

 leen introduced as an impurity of the oxygen gas, it was consi- 

 dered as having formed a part of the combustible gas. A single 

 experiment on any kind of gas was never relied upon ; and to 

 ensure accurate results, the same gas was fired with different 

 proportions of oxygen. Deducting the pure oxygen found in the 

 residue, from its quantity at the outset, the volume of oxygen 

 gas was learned, which had been spent in saturating a given 

 ijaeasure of combustible aas, 



JI*-. Brande's Manual of Cliemislry (p. 156 n.), fi ora which it .nppears that the 

 speedy aclion of chlorine on carbuieUed hvdrogeu had been observed by Mr. 

 Faraday, 



* Th<? Bjethed of doing tins is given ia my Elements of Chemistry, vol. i. chap, t, 

 sect. vi. 



