382 Royal Society: Prof. Johnston on the Constitution 



6, On the Constitution of Pigotite, and on the Mudesous and 

 Mudesic Acids. By James F. W. Johnston, Esq., M.A., F.R.S. 



In this paper the author describes a substance, found by himself 

 and by the Rev. M. Pigot, forming an incrustation on the sides of 

 certain caves, occurring in the granitic cliffs 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 pro- 

 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 pv$r)ats, signifying 

 decay through excess of moisture) ; and he mentions an observation, 

 communicated to him by Dr. Boase, that the roots of the sea-pink 

 (Statice Armeria) 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 C 12 H 5 O 8 . 



2. That this acid, the mudesous, is tribasic, the salt of silver 

 (mudesite), being represented by (3AyO -f C 1<2 H 5 O 8 ), 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 : 



. Dried in the air by (4 Al -f C 12 H a O 8 -f 27 H O). 



b. Dried at 212 F. by (4 Al + C^H, O 8 + 8 H 0), losing 27 

 per cent, of water. 



c. Dried at 300 F. by (4 Al + C ia H 5 O 8 + 8 H O), 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 (Al-j- C 12 H 5 O 8 + U HO) + 3 (Al +6HO). 



b. Dried at 212 F. by (A1 + C 12 H 5 O 8 + 4HO) + 3(A1 + 2HO). 



c. Dried at 300 F. by (Al + C 18 H,,O a + 2HO) + 3 (Al + 2HO), 



