VOL. XXXV.] PHILOSOPHICAL TRANSACTIONS. 249 



these spheres leave any interstices, is found a crocus, or ochre. 3. A stone of 

 iron of the kind used for burnishing plate; it is of the species of the haematites. 

 Both these last stones scrape into a deep crocus. And from the 2d instance we 

 may conjecture, that the yellow colour in crystals arises from a crocus entangled 

 with the stony salts. 



Though the want of wood in Cornwall deprives it of the advantages it might 

 otherwise reap from iron as a metal, we shall nevertheless find it far from being 

 a useless ore, when we consider it as sometimes impregnating the waters with 

 vitriolic salts, thus making them a proper menstruum for dissolving the dis- 

 seminated particles of metals ; sometimes destroying the sulphureous menstrua, 

 which, though they dissolve the disseminated metals, do yet obstruct their new 

 concretions ; and sometimes as being itself the magnet by which the metallic 

 particles are attracted into new concretions. 



Of Tin. — The next metallic substance found in Cornwall, and from which 

 these islands are supposed to take their name, is tin. It is never found but as 

 an ore ; whereas gold is never found but as a metal, at least its ore is unknown, 

 and all other metals are found sometimes as a metal, and sometimes as an ore. 



Tin always shoots into crystals, which are of different magnitudes, from 2 oz. 

 in a single crystal, to such as escape our sight. These crystals are for the most 

 part interspersed in loads of other substances. As, 1. Tin crystals interspersed 

 in a load of a kind of clay, in which is observable a considerable quantity of 

 red-ochre. 2. A kind of hard iron-stone, in which are exceeding small 

 crystals of tin. 3. Somewhat larger crystals, interspersed in a dry red-ochre. 

 4. Tin crystals, interspersed with spar-stone and a sort of marl. 5. Larger 

 crystals, interspersed in a kind of clay and red-ochre, as in N° 1. 



When 100 sacks of the load, each containing more than a Winchester 

 bushel, yield one gallon of clean ore, the load is esteemed very well worth 

 working. Sometimes these crystals are so collected into one mass, as to form 

 loads of pure tin ore, and so large as to yield to the value of lOOl. every 24 

 hours. As, 1. Stones of such pure loads, in which the one is black, and the 

 other nearly white. 



These crystals concrete sometimes into the form of a parallelopepidon, whose 

 summit is covered by a pyramid ; sometimes the angles formed by the sides of 

 the pyramid, and sometimes the summit of the pyramid are as it were planed 

 away. As, 2. A whole crystal, which has none of its angles off; as represented 

 fig. 11, pi. 4. Also a crystal which has only two of its angles planed away ; as 

 fig. 12. And a crystal which has all its angles planed away; as fig. 13. Again, 

 a crystal which has all its angles and its summit planed away ; as fig. 14. 



VOL. VII, K K 



