146 



Copies of the fragments unrolled by Sir Humphry Davy, being 

 fac-similes of the original imitations, executed by Sir William Gell, 

 are annexed to this communication. 



Observations on Naphthaline, a peculiar Substance resembling a Con- 

 crete Essential Oil, which is apparently produced during the Decom- 

 position of Coal Tar, by Exposure to a Red Heat. By J. Kidd, M.D. 

 Professor of Chemistry, Oxford. Communicated by W. H. Wol- 

 laston, M.D. F.R.S. Read March 8, 1821. [Phil. Trans. 1821, 

 p. 209.] 



By passing coal tar through a red-hot iron tube, a portion of an 

 aqueous fluid, and of a substance like tar, was obtained ; the latter 

 is black, soluble in ether, and partially in alcohol, of an aromatic 

 odour, and sweetish taste. It was submitted to slaw distillation, 

 and among other products afforded naphthaline, a white concrete sub- 

 stance of an aromatic odour and taste, fusible at 180, and scarcely 

 soluble in water, but readily so in ether, alcohol, and oils. 



Of the various characters of this substance, detailed by the author, 

 its tendency to crystallize appears the most remarkable ; its vapour 

 condenses in rhombic plates, which are sometimes modified into hex- 

 agonal plates, by the incomplete development of the smaller angles 

 of the usual rhomb. 



The other substances obtained along with the naphthaline were 

 ammoniacal water, and an oil of a bituminous and aromatic odour, 

 boiling at 210, and not congealing at 32, highly inflammable, and 

 readily soluble in alcohol and ether : there was also produced, during 

 the latter part of the distillation, a yellow farina precipitable from 

 its alcoholic solution by water, and fusible. Of these four substances, 

 resulting from the distillation of the black liquid obtained by distil- 

 ling coal tar, Dr. Kidd thinks that the water and the farina are pro- 

 ducts, and the other mere educts of the operation. 



On the Aberrations of Compound Lenses and Object -Glasses. By 

 J. F. W. Herschel, Esq. F.R.S. *e. Read March 22, 1821. [Phil. 

 Trans. 1821, p. 222.] 



To those mathematicians who have investigated the theory of the 

 refracting telescope, it has often, says Mr. Herschel, been objected, 

 that little practical benefit has resulted from their speculations. Al- 

 though the simplest considerations suffice for correcting that part of 

 the aberration which arises from the different refrangibility of the 

 different coloured rays, yet in the more difficult part of the theory of 

 optical instruments which relates to the correction of the spherical 

 aberration, the necessity of algebraic investigation has always been 

 acknowledged ; although, however, the subject is confessedly within 

 its reach, a variety of causes have interfered with its successful pro- 

 secution, and the best artists are content to work their glasses by 

 empirical rules. In the investigations detailed in this paper, the 



