92 CONSTRUCTION OF RETORTS. 



establishment, which have been in operation upwards of seven years, remain 

 perfectly sound, and continue as efficient for making gas as on the first day 

 they were at work. 



At my request Mr. Grafton has favoured me with the following account of 

 the cause of carbonaceous deposit in retorts : 



"After a series of experiments established in 1839 at the Cambridge Gas-works, and 

 after having in vain offered a large premium for the discovery, I was myself enabled to 

 detect the origin of the great accumulation of the carbonaceous deposit in coal-gas re- 

 torts, as well as the means of obviating an evil which has been the source of so much 

 loss to the manufacturers of gas. Previously to that period the most eminent scientific 

 authorities consulted on the subject considered this accumulation as the result of high 

 degrees of heat and too great an extent of heated surface. 



" To ascertain how far this opinion was correct, I commenced my experiments with a 

 number of retorts reduced to various lengths, by the ends being filled up with brick- 

 work, the other dimensions remaining unaltered. 



" By this difference of length, after repeated trials and at various temperatures, the 

 deposit did not appear to be diminished, although it did not accumulate so rapidly ; and 

 finally it formed a coating of the same substance, not less in a short retort than in a 

 long one. 



" It was observed in all cases that the substance began to form itself first at the closed 

 end of the retort, whence it gradually advanced and accumulated in bulk ; at that end 

 the coal (especially with cylindrical iron retorts) is carbonized first ; hence the inference 

 is, that the best constituents, viz. the hydro-carburets, being without the means of 

 escaping, become decomposed, leaving as a result the carbonaceous deposit. 



" I then had two retorts constructed with an ascending pipe to carry off the gas at 

 each end, so that its stream might divide itself in equal portions each way, thus reducing 

 its passage over the heated surface from seven feet (the length of the retorts) to three 

 feet six inches, and affording equal means of escape to the gas from all parts of the coal. 

 The deposit after three months of constant working was considerably less at the closed 

 end of the retort ; but it formed itself in the same quantities on the roof, and soon covered 

 the whole of the inner surface, gradually, as heretofore, diminishing the capacity of the 

 retorts and increasing the consumption of fuel. 



" The resistance offered to the gas during these experiments by the purifiers and the 

 weight of the gas-holders, was equal to a column of water of nine inches by the gauge on 

 the mouth-piece ; this pressure having varied by the alteration of the weight of the gas- 

 holders between wdnter and summer. I remarked that the accumulation was not so rapid 

 in the summer months, when the resistance was less and the gas less compressed. I im- 

 mediately had the pressure increased to a column of fourteen on the gauge, keeping up the 

 usual heat. The retort for this experiment, like all the rest, w as constructed of fire-bricks 



