14 NATURAL HISTORY 



LETTER IV. 



TO THE SAME. 



As in a former letter the freestone of this place has 

 been only mentioned incidentally, I shall here become 

 more particular. 



This stone is in great request for hearth-stones, and 

 the beds of ovens ; and in lining of lime-kilns it turns 

 to good account : for the workmen use sandy loam 

 instead of mortar; the sand of which fluxes ! , and runs 

 by the intense heat, and so cases over the whole face 

 of the kiln with a strong vitrified coat like glass, that it 

 is well preserved from injuries of weather, and endures 

 thirty or forty years. When chiselled smooth, it makes 

 elegant fronts for houses, equal in colour and grain to 

 the Bath stone; and superior in one respect, that, 

 when seasoned, it does not scale. Decent chimney- 

 pieces are worked from it of much closer and finer 

 grain than Portland; and rooms are floored with it; 

 but it proves rather too soft for this purpose. It is a 

 freestone, cutting in all directions ; yet has something 

 of a grain parallel with the horizon, and therefore 

 should not be surbedded, but laid in the same position 

 that it grows in the quarry 2 . On the ground abroad 

 this firestone will not succeed for pavements, because, 

 probably, some degree of saltness prevailing within it, 

 the rain tears the slabs to pieces 3 . Though this stone 

 is too hard to be acted on by vinegar, yet both the 

 white part, and even the blue rag, ferment strongly in 

 mineral acids. Though the white stone will not bear 



1 There may probably be also in the chalk itself, that is burnt for lime, 

 a proportion of sand ; for few chalks are so pure as to have none. 



3 " To surbed stone is to set it edgewise, contrary to the posture it had 

 in the quarry," says Dr. Plot, Oxfordsh. p. 77. But surbedding does not 

 succeed in our dry walls; neither do we use it so in ovens, though he 

 says it is best for Teyuton stone. 



3 Firestone is full of salts, and has no sulphur: must be close-grained, 

 and have no interstices. Nothing supports fire like salts; salt-stone 

 perishes exposed to wet and frost. I'lot's Xt'ijf. p. 152. 



