1868.] On the Knowledge of Per sulphide of Hydrogen. 4^^7 



to it, the third parallel to the polar axis. They are so effective, that 5 lbs. 

 at a leverage of 20 feet turns the polar axis ; 12|- the declination one. A 

 man can raise the telescope from the horizon to the zenith in 20 seconds ; 

 two (as both axes must be turned) can reverse it from the east side of the 

 pier to the west in 45 seconds. 



The telescope is moved in right ascension by a sector and screw driven 

 by a very effective clock. The regulator of this is so powerful, that an 

 addition of 2 cwt. to its driving-weight only makes it gain six seconds in 

 the hour. 



The micrometer has an original mode of illuminating its lines in a dark 

 field, which has been found very suitable for nebulae. 



The spectroscope is on the usual plan, but with special provision for the 

 permanence of its adjustments. 



The instrument is also provided with a photographic apparatus, nearly 

 like Mr. De la Rue's celebrated one, which (the small speculum being 

 removed) is placed at the focus of the great speculum. A few trials 

 made with an extemporized one gave pictures which that gentleman con- 

 sidered to be of great promise. 



A high opinion is expressed both of the optical and astronomical 

 powers of the instrument. 



June 18, 1868. 

 Lieut. -General SABINE, President, in the Chair. 



Prof. Clifton, Dr. J. Barnard Davis, Dr. Duncan, Dr. Pettigrew, Mr. 

 Stone and Mr. Vaux, were admitted into the Society. 



The following communications were read : — 



I. "A. Contribution to the Knowledge of Persulphide of Hydrogen.*' 

 By A. W. HoFMANN, LL.D., E.R.S. Received May 25^1868. 



This remarkable body was first observed by Scheele, and subsequently 

 examined by Berthollet ; our knowledge of this substance is, however, 

 more especially due to Thenard, who, soon after the discovery of peroxide 

 of hydrogen, was led also to investigate what was believed to be the cor- 

 responding sulphur-compound *. The composition of persulphide of hy- 

 drogen has nevertheless remained doubtful. Thenard points out that the 

 specimens analyzed by him, contained variable quantities of sulphur, but 

 always more than would have been met with in a sulphur-compound cor- 

 responding to peroxide of hydrogenf . 



^ Aim. de China, et de Phys. vol. xlviii. p. 79. 



i" Thenard states that all his analyses yielded more than 4 atoms of sulphur for 1 

 molecule of sulphuretted hydrogen. 



2 a 2 



