Mr. Woods on the Anthracite Coal of South Wales. 211 



are they restored by a liquid hyposulphite ; the paper, in- 

 stead of being completely white, being altogether of a full 

 rich yellow. When these photographs are restored by the 

 hyposulphite, they are even less permanent under the influence 

 of light than those washed with the salt of lead. 



35. The ferrocyanate of potassa exerts no action on these 

 photographs in any way remarkable, unless they have been 

 formed by the agency of the hydriodate of iron (22.) or of 

 baryta and iron (27.). They are then obliterated by it, but 

 on exposure, the light parts of the picture are darkened, 

 changing thus to a negative photograph, the originally dark 

 parts being now a light blue. 



36. With much attention, I have tried the hyposulphites 

 of soda, ammonia and potassa. But I have failed to remove 

 all the iodide of silver, without destroying at the same time 

 the dark parts, and the minute portion which remains in the 

 paper is very soon darkened by light to a tint similar to the 

 lighter shades of Indian ink. When first done the drawing 

 is much improved in appearance, but it is difficult to remove 

 the hyposulphite so completely as is necessary to prevent the 

 formation of the sulphuret of silver. 



37. Sulphuretted hydrogen gas, which has the singular 

 property of blackening the iodide of silver, when in that state 

 which is easily darkened by light, hut of bleaching it in the less 

 susceptible state, acts on these photographs in a manner simi- 

 lar to the hyposulphites; but the oxidized portions of the 

 picture are first destroyed and then restored by light. The 

 light parts are, however, rendered brown. 



I have tried a great variety of other agents, diversifying 

 my method of using them in almost every possible way, but 

 as yet I have discovered no material which effectually removes 

 the iodide of silver alone; consequently I satisfy myself with 

 well washing my photographs in hot water. 

 [To be continued.] 



XXX. On the Anthracite Coal of South Wales. By Samuel 

 Woods, Esq., F.G.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



AVING received from a friend at Paris the accompany- 

 ing analysis of some of the anthracite coal found abun- 

 dantly in this neighbourhood, I transmit it to you in the 

 belief that it may be worthy of record in your pages; and the 

 quality and uses of this coal having lately excited so much 



P2 



H 



