70 REPORT — 1855. 



a discovery made originally by M. Niepce of Chalons. 2. A negative on glass, or 

 waxed-paper, is applied to the sensitive coating of bitumen, and exposed to the full 

 rays of the sun for a period longer or shorter according to the intensity of the light, 

 and a faint impression on the bitumen is thus obtained. 3. The stone is now placed 

 in a bath of sulphuric aether, which almost instantaneously dissolves the bitumen, which 

 has not been acted upon by light, leaving a delicate picture on the stone, composed of 

 bitumen on which the light has fallen. 4. The stone, after being carefully washed, may 

 be at once placed in the hands of the lithographer, who is to treat it in the ordinary 

 manner with gum and acid, after which proofs maybe thrown off by the usual process. 

 Prof. Ramsay then proceeded to state, that the above process, modified, had been 

 employed with success to etch plates of steel or copper, without the use of the 

 burin : — 1. The metal plate is prepared with a coating of bitumen, precisely in the 

 manner noticed above. 2. A positive picture on glass ov paper is then applied to the 

 bitumen, and an impression is obtained by exposure to light. 3. The plate is placed 

 in a bath of aether, and the bitumen not acted upon by light is dissolved out. A beau- 

 tiful negative remains on the plate. 4. The plate is now to be plunged into a galvano- 

 plastic bath, and gilded. The gold adheres to the bare metal, but refuses to attach 

 itself to the bitumen. 5. The bitumen is now removed entirely by the action of spirits 

 and gentle heat. The lines of the negative picture are now represented in bare steel 

 or copper, the rest of the plate being covered by a coating of gold. 6. Nitric acid is 

 now applied as in the common etching process. The acid attacks the lines of the 

 picture formed by the bare metal, but will not bite into the gilded surface. A perfect 

 etching is thus obtained. 



On the Composition of Vandyke-Broion. 



By Thos. H. Rowney, Ph.D., F.C.S. 

 This pigment is of organic origin, and is obtained from the peat beds in Cassel in 

 Germany. It is a brown earthy-looking substance, a little heavier than water. It 

 was found to be an organic acid with abo>it 6-00 of earthy matter. The formula 

 deduced from the analyses is C54H20 O24. It is very soluble in alkaline solutions, and 

 forms salts with various metals and alkaline earths. Being a distinct mineral, the 

 name Vandykite is proposed for it. 



On the Composition of two Mineral Substances employed as Pigments. 

 By Thos. H. Rowney, Ph.D., F.C.S. 



In this communication two new minerals are described which have for some con- 

 siderable time been employed as pigment, but had not previously been described. 

 The first, colled Indian red, is brought from the Persian Gulf. It occurs as a 

 coarse powder of a deep red colour; its sp. gr. is 3-843. By analysis it was found 

 to be a silicate of iron, having the formula FcjOg-J-SiOg. This corresponds in con- 

 stitution to xenolite, which is a silicate of alumina of the formula Alj O3 -j- SiOj. 



The second mineral, called raw sienna, is obtained from Sienna. It is a soft 

 earthy substance, of a brownish-yellow colour; its sp. gr. is 3'46. It is hydrated 

 silicate of iron containing a small quantity of alumina, and has the formula 

 4(Fe203, Al203)-f-Si03-|-6HO. The name proposed for it is Hypoxanthite ; in con- 

 stitution it resembles opaline allophane, and Schrbtterite. 



Hypoxanthite 4(Fe2 03, AlgOg) -f SiOg-l- 6HO 



Opaline allophane ... 4AI2O3 -l-SiOs-HlSHO 



Schrbtterite 4AI3O3 -f SiOg-f 16H0. 



On certain Laws observed in the mutual action of Sulphuric Acid and Water, 

 By Balfour Stewart. 



The object of this paper is to show that in mixtures of sulphuric acid and water 

 there is a distinct dependence on tho chemical equivalents of these substances, and 

 several hydrates are indicated. 



The method of analysis used is applicable to other solutions. 



When sulphuric acid combines with water the space occupied by the compound is 

 less than that occupied by the ingredients when uncombined, and consequently the 



