23 



probably astronomical rather than geological, and are to be found in 

 the changes in the ellipticity of the earth's orbit, in the alteration in 

 the position of the earth's axis, and in the variation in the obliquity 

 of the ecliptic. 



TUESDAY, NOVEMBER 2nd. 

 Dr. Aemstbong, F.R.S., Sec. C.S., delivered a Lecture on 



"Coal Gas." 



In the manufacture of gas, coal is heated at a high temperature 

 in vessels known as "retorts;" decomposition ensues and a large 

 amount of "gas" is given off, together with numerous hquid and 

 sohd substances in a state of vapour. From the retorts, this 

 mixture of gases and vapours passes through tubes into the 

 "hydraulic main," and thence to the "condensers," where, owing to 

 the reduction in temperature, the chief part of the liquid and 

 solid matters are deposited. The gas issuing from the condensers 

 go^s through the "scrubbers," which are towers filled with coke 

 over which water or weak ammoniacal liquor constantly trickles ; 

 it there becomes thoroughly cooled and is deprived of ammonia. 

 Lastly, in order to free it from sulphuretted hydrogen and carbon 

 dioxide, it is passed through "purifiers" containing oxide of iron 



and slaked lime. 



Notwithstanding the purification which the gas undergoes, it is 

 still contaminated with a small amount of sulphur compounds 

 and usually also contains traces of ammonia. When the gas is bm-nt, 

 these become converted into sulphuric and nitric acids, which are 

 extremely coiTOsive. The Lecturer considered that the public ought 

 to insist on further efforts being made by the gas companies to 

 remove these impurities, as they are the means of effecting the 

 most serious damage to furniture, pictures, books, &c. 



The matters deposited in the condensers consist of an aqueous 

 portion— the so-called ammoniacal or gas liquor; and of an oily 

 portion-the well-known coal-tar. The ammoniacal liquor is of 

 value as a source of ammonia salts, many thouands of tons of 

 ammonium sulphate being annually manufactured from it, and 



