THE HISTORY OF GEOLOGY AT OXFORD. 35 



After his exposition of the weak points in Steno's argu- 

 ment Dr. Plot proceeds to consider the alternative view, 

 that the formed stones are the result of some "plastic 

 force " ; and as later writers have made merry over this 

 expression, it will be merest justice to explain what Dr. 

 Plot meant by it. Whatever the plastic force of Theo- 

 phrastus may have been, that of Dr. Plot was certainly 

 what we now recognise as crystallisation. His words are : 

 " There is no other Principle that we yet know of naturally 

 shooting into Figures, each peculiar to their own kind, but 

 Salts, thus Nitre always shoots into Pyramids, salt Marine 

 into Cubes, Alum into Octo, and Sal Ammoniac into Hexa- 

 hedruuis, and other mixt Salts into as mixt Figures. 



" Of these spontaneous inclinations of Salts each peculiar 

 to its Kind, we have further evidence in the Chymical 

 Anatomy of Animals, particularly in the Volatile Salt of 

 Harts-horn, which in the Beginning of its Ascent is always 

 seen branched in the Head of the Cucurbit, like the Natural 

 Horn. And we are told by the very ingenious and learned 

 Sidleyiau Professor here in Oxon, That the Salt of Vipers 

 ascends in like manner, and shoots into Shapes somewhat 

 like those of Animals, placed orderly in the Glass. Thus 

 in Congelations, which are all wrought by adventitious 

 Salts, we frequently find curious Ramifications, as on Glass- 

 windows in Winter, and the figured Flakes of Snow, of which 

 Mr. Hook observed above an Hundred several sorts, yet all 

 of them branched as we paint Stars, with six principal 

 Radii of equal Length, Shape and Make, issuing from a 

 Center, where they all joined in Angles of 60 Degrees. 



"What Salt it should be that gives this Figure, though 

 it be hard to determine, yet certainly it must not be a much 

 different one from that which gives Form to our Astroites 

 and AstericE, whereof, though the latter have but five 

 Points, and therefore making Angles where they are joined 

 at the Center of 72 Degrees: yet the Astroites both in 

 mezzo Rilievo and Intagli have many more. 



" Perhaps there may be something of an Antimonial Salt 

 that may determine Bodies to this Starry Figure, as no 

 Question it does in the Regulus, and the Caput mortimm of 



