410 MIXED GASES. 



(d.) Wigan coal has been esteemed the hest in England ; the gas 

 from this coal, required, on an average, only 155 volumes of oxygen 

 to 100 of the coal gas, and gave 88 measures of carbonic acid. 



(e.) As the inflammable gases produce light just in proportion to 

 the quantity of carbon they contain, it follows, that oil gas is nearly 

 or quite as powerful as gas from Wigan coal. 



3. MISCELLANEOUS. 



(a.) Mr. Brande estimates, that to produce a quantity of light 

 equal to that of ten wax candles, burning for one hour, there are re- 

 quired 2600 cubical inches of olefiant gas, 4875 of oil gas and 13120 

 of coal gas. 



(b.) Dr. Henry suggests that this estimate is, as regards coal gas, 

 rather low, and is disposed to consider 1 volume of oil gas as equiva- 

 lent to 2 or 2J of coal gas. 



(c.) The late Mr. Creighton, of Glasgow, considered 2 volumes 

 good coal gas as equal, in affording light, to only one of oil gas, and 

 valuing the quantity of light given by one pound of spermaceti candles 

 at 1 shilling, he estimated the cost of an equal effect from sperm oil, 

 burning in an Argand's lamp, at 6 JJ. that from whale oil at 4jc?. and 

 that from coal gas at 2jdf. " Twenty cubic feet of coal gas, or ten 

 of oil gas, he considers as equivalent to a pound of tallow, and 5000 

 grains of spermaceti oil to 7000 of tallow or 1 Ib. avoirdupois." 



(d.) Dr. Henry sums up the comparative claims of oil and coal 

 gas, by saying, that for oil gas, vessels and tubes of half the size are 

 sufficient ;* no washing is needed ; there is no residuum ; the light is 

 brighter and the heat less ; but that still, in large establishments and 

 in countries where coal is cheap, the latter will be preferred on the 

 score of economy. 



(e.) The best criterion of the illuminating power of a gas is the 

 quantity of oxygen required for its perfect combustion,! and the 

 amount of carbonic acid produced ; specific gravity is deceptive, for 

 it may be affected by foreign gases, for instance, by carbonic oxide 

 or by carbonic acid. J 



(/.) It appears that a very valuable illuminating gas is obtained by 

 decomposing cotton seed by a well managed heat. Prof. Olmsted 

 lias shewn that it is both economical and effectual^ 



(g.) Coal gas is obtained by decomposing coal in an iron retort ; 

 the tar is received in a condensing vessel, and more continues to be 

 deposited by the passage of the gas through vertical tubes kept cold. 

 The gas, under strong pressure, is passed through lime diffused in 



* Oil gas, being; free from sulphuretted hydrogen, needs no purification, and is 

 therefore peculiarly fitted for dometic use. 



t It is suggested that condensation by chlorine niay be a test equally decisive. 

 Comm. 



I Henry's Chem. Vol. I, p. 432, 10th ed. 



Am, Jour, Vol. XIII, p. 194, and Vol. X. 



