Page:Proceedings of the Royal Society of London Vol 4.djvu/266

Rh and by the Rev. M. Pigot, forming an incrustation on the sides of certain caves, occurring in the granitic clilFs on the east and west coast of Cornwall. This incrustation is in mass of a brown, and in powder of a yellow colour ; is insoluble in water and alcohol ; when heated, it gives off much water, blackens, yields empyreumatic jDro- ducts, and leaves a black mass, having occasionally the lustre of graphite. In the air, at a bright red heat, this mass very slowly burns, leaving a grey or white ash, which consists of alumina, with some slight foreign admixtures.

The organic constituent of this substance (pigotite), the author considers to be derived from the decay of the various plants which grow on the moist moorlands above, and which, being carried by the waters into the fissures of the granite beneath, combines with the alumina of the decomposed felspar ; and when it reaches the air, deposits itself on the roof and sides of the caverns, in the form of layers, varying from a line to two or three inches in thickness. With reference to its supposed origin, the author has given to the organic constituent the name of mudesous acid (from fivdrjais, signifying decay through excess of moisture), and he mentions an observation, communicated to him by Dr. Bouse, that the roots of the sea pink (Statice armerid) contain a colouring matter resembling, in appear- ance, the solutions of the mudesous acid.

From numerous experiments and analyses detailed at length in his paper, the author derives the following general results :

1 . That the native pigotite contains a dark-brown soluble, not deliquescent acid of vegetable origin, which, in the anhydrous state, is represented by Cjo H5 Og.

2. That this acid, the mudesous, is tribasic, the salt of silver (mudesite), being represented by (3 Ag O + Ci^HjOg), and pre- cipitates the salts of the metallic oxides of a brown colour.

3. That the native mudesite of alumina (Pigotite) is repre- sented as follows :

a. Dried in the air by (4 Al -}- C.^H-^ Og 27 H O).

b. Dried at 212« F. by (4 + C.^H^ Og + 8 H O), losing 27 per cent, of water.

c. Dried at 300« F. by (4 Al + H, 0, 4- 8 HO), losing 32 per cent, of water.

4. That this native mudesite, however, is more probably a com- pound of the organic tribasic salt, with a hydrate of alumina, and may be rationally represented thus :

a. Dried in the air by {M + Cj.HsOs + gHO) + 3 (Al + 6H0).

b. Driedat212«F.by(Ai_+C,2H508 + 4HO) + 3(A1 + 2H0).

c. Driedat300«F.by (^ + C,„H3 0g-}-2HO) + 3(Al4- 2H0).

5. That when treated with nitric acid, the native mudesite, as