408 MIXED GASES. 



MIXED GASES ; obtained by heating various combustible bodies, 

 as tallow, alcohol, ether, bituminous coal, &ic. 



Remark. Although, as has been already observed, these gases 

 are, in all probability, mixtures of the varieties that have been de- 

 scribed, they do, in practice, present some peculiarities worthy of be- 

 ing noted. 



I. COAL GAS. 



(a.) There is so much variety in the properties of the gases ob- 

 tained by heating mineral coal, that they are hardly worthy of being 

 grouped together, except on the ground that they are obtained from 

 a common material. 



(6.) Bituminous coal, distilled in an iron retort, affords, besides the 

 permanent gases, tar and solution of carbonate of ammonia. 



(c.) The gas varies in quality, even from the same coal, at dif- 

 ferent stages of the process, according to the degree of heat and the 

 manner of applying it ; of course, it varies with different specimens 

 of coal. 



(d.) Dr. Henry remarks, "within certain limits, the more quickly 

 the heat is applied, the greater is the quantity and the better the qual- 

 ity, of the gas obtained from coal ; for, too slow a heat expels the 

 inflammable matter in the form of tar."* 



(e.) The gas declines much in quality towards the end of the oper- 

 ation, although we still continue to obtain large quantities. 



K) The useful part of the gas is composed of mixtures of light 

 leavy carburetted hydrogen, in endlessly varied proportions. 



(g.) The useless gases are carbonic acid, oxide of carbon, nitro- 

 gen and sulphuretted hydrogen,f and sometimes ammonia, (and sul- 

 phurous acid gas?) 



(h.) The disagreeable smell arising from sulphuretted hydrogen, 

 and probably a little sulphuret of carbon, may be washed out by cream 

 of lime, without injuring the combustibility of the gas. 



(i.) The best gas has the sp. gr. of at least 650, air being 1, " and 

 each volume consumes about 2J volumes of oxygen and gives 1 J vol- 

 ume of carbonic acid." 



(/.) " The last portions have a sp. gr. as low as .340, and each 

 volume consumes about .8 of a volume of oxygen gas and gives about 

 .3 of a volume of carbonic acid." 



(k.) Chlorine, applied in a manner hereafter to be pointed out, 

 detects from 13 to 20 per cent of olefiant gas; the rest is chiefly light 

 carburetted hydrogen. 



* Phil. Trans. 1808, 1820, 1824. 



t Dr. Henry refers us, for the method of separating them, to his memoirs above 

 quoted, to Manchester Memoirs, and Annals of Philosophy, XV. 



