A DISCOURSE

IQ

ras it does not replenish the fofs out of which it has been dug, the cornwhich is sown in that country soon degenerates into rye; and what is stillmore remarkable, that the rye sown in Thuringia (where the earth is lefscompacted) reverts, after three crops, to be wheat again f .

My Lord Bacon directs to the observation of the Rainbow, where itsextremity seems to rest, as pointing to a more roscid and fertile Mould ;but this, I conceive, may be very fallacious, it having two horns, orbases, which are ever opposite.

But the situation and declivity of the place is commonly a more certainmark ; as what lies under a Southern, or South-east rising ground ; but

they 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 portionof argillaceous Earth .

As Stones are intimately connected with Earth , I shall in this place give a genera]idea of them, in order that the reader may have a full and comprehensive view olthat mixed body', which is the subject of this Efsay.

Stones may be comprehended under the four following genera.:

I. Calcareous , or Lime Stones.

1 . These effervesce with, and difsolve in, the mineral acids; and in the fire burn tolime.-

II. Argillaceous, or Clay Stones.

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

III. Gy/isecus, or Plaister Stones.

1. These are not affected by acids. They burn to plaister in the fire, and beingwetted with water, presently grow hard, in which they differ from lime, whichdoes not harden upon wetting, unlels mixed with sand, and not then till afteralong time.

IV. Vitrescent, or Glafs Stones.

1. These suffer no change with acids, and in the fire run to glafs. All this genusstrike fire with steel, except the glafs spar and the pumic.e stone.

%* A great many small, visible, vitrescent Stones, constitute what is called Sand , whichis either coarse or fine. Sometimes Sand consists of one species only, but oftener oftwo or more. When these cohere, they -form the Grit or Sand Stone , of which Mill-Stones, Grind-Stones , &c. are made.

f This observation of Laurembergius has neither reason nor experiment to recommendit. It is iu the last degree absurd.