10 



A DISCOURSE 



as it does not replenish the foss out of which it has been dug, the corn 

 which is sown in that country soon degenerates into rye ; and what is still 

 more remarkable, that the rye sown in Thuringia (where the earth is less 

 compacted) reverts, after three crops, to be wheat again ^ 



My lord Bacon directs to the observation of the rainbow, where its 

 extremity seems to rest, as pointing to a more roscid and fertile mould ; 

 but this, I conceive, may be very fallacious, it having two horns, or 

 bases, which are ever opposite. 



But the situation and declivity of the place is commonly a more certain 

 mark ; as what lies under a southern, or south-east rising ground ; but 



are pretty hard, but being exposed to the air, soon fall into powder. They are of various 

 colours, but are seldom pure, being commonly mixed with a portion of argillaceous 

 earth. 



*^* As slmes are intimately connected with earths, I shall, in this place, give a general idea 

 of them, in order that the reader may have a full and comprehensive view of that mixed 

 body, which is the subject of this Essay. 



Stones may be comprehended under the four following genera : 



Calcareous, or lime stones. 



1. These effervesce with, and dissolve in, the mineral acids; and in the fire burn to 

 lime. 



II. Argillaceous, or clat/ stones. 



1. These are insoluble in acids, and burn to hardness in the fire. 



III. Gypseous, ov plaster stones. 



1. These are not affected by acids. They burn to plaster in the fire, and being 

 wetted with water, presently grow hard, in which they differ from lime, which 

 does not harden upon wetting, unless mixed with sand, and not then till after 

 a long time. 



IV. Vitrescent, or glass stones. 



1. These suffer no change with acids, and in the fire run to glass. All this genus 

 strike fire with steel, except the glass spar and the pumice stone. 

 A great many small, visible^ vitrescent stones, constitute what is called sand, which 

 is either coarse or fine. Sometimes sand consists of one species only, but oftener of two 

 or more. When these cohere, they form the grit or sand stone, of which mill-stones, 

 grind-stones, &c. are made. 



^ This observation of Laurembergius has neither reason nor experimait to recommend it^ 

 It is in the last degree absurd. 



