79 



protoxide is exceedingly hard, it does not protect the interior 

 of the iron ; the process of destruction is slow but certain. 

 Earthen gas retorts, such as were patented several years since 

 by Mr. Grafton, would perhaps answer better. 



*' On making a trial of this, which has been called water gas, 

 I found that, notwithstanding the presence of much carbonic 

 oxide, it gave off quite a sufficiency of heat, when burned in 

 my different lamps, to volatilize the hydro-carbon, and to afford 

 a brilliant light: by itself it burns blue. 



" The various volatile combustible liquids already men- 

 tioned are applicable to the foregoing purposes with different 

 degrees of efficacy. Those that require high temperatures for 

 their volatilization are the fittest, because the source of heat 

 is so near the wire-gauze that the vaporization easily takes 

 place. 



" But there are hydro-carbons which, being volatilized at 

 the ordinary temperature of the air, afford much more manage- 

 able means of rendering hydrogen gas luminous in burning. 

 Of these, by far the most convenient and economical are the 

 finer kinds of coal naphtha. It is obtained by the distillation 

 of coal tar, and is procurable in various degrees of purity in 

 commerce. Coal tar is an article of very small price, and when 

 deprived of its naphtha, instead of being depreciated in value, 

 it is enhanced. By several rectifications the naphtha becomes 

 pure and colourless, and very volatile. Its quality varies ma- 

 terially according to the coal used at the gas works from which 

 the tar was obtained. The greater the specific gravity, pro- 

 vided it is pure, the greater is its volatility. The purest and 

 heaviest naphtha that I have been ever able to obtain from 

 Newcastle coal tar, was s. g. 0-888 : this was the result of 

 repeated rectifications, the first products only, in every case, 

 being retained. This naphtha began to boil at 166°, but 

 boiled rapidly at 172°. The purest and heaviest naphtha, 

 from Scotch Parrot coal tar, that I could procure by any num- 

 ber of rectifications, was s. g. 0862° ; it began to boil at 183°, 



