PROCEEDINGS OP THE POLYTECHNIC ASSOCIATION. 671 



their passage to the burner. Thus naphthaline, C20 Hs, a solid, having 

 only one half the hydrogen found in the turpentine series, emits in the pro- 

 cess of coal distillation, a considerable quantity of vapor, causing, to a 

 great degree, the peculiar odor of coal gas, which, as it cools, is gradually 

 condensed and deposited in the street mains. This action causes serious 

 inconvenience, by narrowing the channel of the pipes and preventing the 

 passage of the requisite supply of gas. The only preventive of this evil is 

 to thoroughly cool the gas before it leaves the gasometer, or to pass it over 

 coal oil, which will absorb, to a certain extent, the naphthaline. 



In the distillation of coal, the use of too high a degree of heat should be 

 avoided, because the requisite decompositions are best obtained by low 

 temperattires. The effect of too high heat is to abstract the carbon from 

 the hydrogen, giving, as the principal resultants of distillation, carbonic 

 oxide, hydrogen and coke. The use of a low temperature is the chief pe- 

 culiarity in the process of producing oil from cannel coal. 



It is important that the gaseous products should not remain too long ex- 

 posed to the heat of tlie retorts. Their passage is expedited and their qua- 

 lity improved by mixing in the current the compounds which result from 

 the decomposition of steam when it is passed over red hot coals. 



The purification of the gas is effected chiefly by caustic lime, which ab- 

 sorbs the sulphuretted hydrogen and carbonic acid, forming sulphide of cal- 

 cium, water, and carbonate of lime. 



A mixture of the hydrated peroxide of iron and slaked lime and saw 

 dust, has been substituted in many manufactories in the place of caustio 

 lime. The oxide of iron unites with the sulphur in sulphuretted hydrogen 

 and sulphide of ammonium, while the slaked lime absorbs carbonic acid 

 and carbonate of ammonia, and is converted into the subcarbonate of lime. 

 The object in using saw dust is to make the mixture more porous, thereby 

 allowing it to be more readily penetrated by gaseous products. No sub- 

 stance has been found to arrest the bisulphide of carbon, and probably a 

 certain portion of sulphuretted hydrogen; these compounds are, to a small 

 extent, carried to the burner, and, by combustion, sulphurous acid is pro- 

 duced. This dereterious gas is mixed with the air we breathe when using 

 coal gas, and is the most serious objection to this kind of illumination. As 

 bisulphide of carbon is a resultant of higli heat, it seems far more feasible 

 to make the gas at a low temperature, and thus in a great measure prevent 

 the sulphide formation. Sulphur when united with carbon, is the inevitable 

 bane that accompanies all the blessings we enjoy by means of coal gas 

 illumination. The inventor who can devise a plan by which only pure hydro- 

 carbon gases, in proper proportions, will flow into our domiciles for lumi- 

 niferous uses, will deserve the general gratitude and an ample fortune. 



Dr. Rich. — The quality of the gas manufactured in this city varies, but 

 the average of the illuminating power of a four foot burner is equal to the 

 light of fourteen and a half candles, the gas burning at the rate of one 

 cent an hour. 



Dr. R. P. Stevens. — In the great law case some few years ago in regard 

 to the so called Albert coal, it was then stated by chemists that about 

 seventy-two different products had been taken from coal by distillation, 

 and the number could be vastly increased. I believe it will be a long timo 



