THE PRODROMUS 241 



3. That when oysters, mussels, and other bodies have de- 

 composed within the earth, the cavities are filled with crystals. 



The movement of the crystalline matter toward a point 

 where the planes of the crystal already formed are fixed, does 

 not arise from some common cause of motion in the surround- 

 mg fluid, but varies in any given crystal ; so that in reality it 

 depends upon the movement of the tenuous fluid flowing from 

 the crystal already formed, and the result is: 



1. That in the same place crystalline matter is added to 

 planes which face the horizon from different angles. 



2. That in the same fluid crystals of different shapes are 

 produced. Whether the fluid is that by the aid of which re- 

 fraction is caused, or there is still some fluid different from it, 

 I leave to wiser minds to study. 



That the efficiency of a penetrating fluid is certainly great, 

 is illustrated by the row of iron filings which rise about the 

 poles 1 of a magnet, not only when the filings are in direct con- 

 tact with the magnet but also when they are separated from 

 the magnet by an intervening sheet of paper. When, for ex- 

 ample, the magnet is moved in various ways below the paper, 

 p. 43. while one end remains at rest, filings of this kind describe on 

 the paper all the arcs which can be drawn within a hemisphere. 

 Now all advance from place to place like armed soldiers ; now, 

 deflected by the approach of another magnet, they form an arch 

 just as if the individual parts of the filings had been glued to- 

 gether and had united into a solid body.^ 



^ Poros in the original edition is an obvious error for polos. 



^ This experiment with the magnet was, no doubt, a scholastic commonplace. A curious 

 analogue to Steno's illustration may be found in Robert Boyle's The Effects of Languid 

 Motion Considered (edition of P. Shaw, London, 1725), Vol. I, pp. 477, 478 : 



'' The load-stone is acknowledged to act by the emission of insensible particles. For tho' 

 iron and steel be solid bodies, and magnetic effluvia corpuscles so very minute, as readily to 

 get in at the pores even of glass itself ; yet entring the steel in swarms, they may operate so 

 violently on it, as to attract above fifty times the weight of the magnet. For to these I rather 

 ascribe magnetical attraction and suspension, than to the pressure of the ambient air; because 

 I have found on trial, that such a pressure is not absolutely necessary to magnetical operations. 



" And farther, as to the power of magnetical effluvia upon iron, I took filings of iron fresh 

 made, that the virtue might not be diminished by rust, and having laid them in a little heap 

 upon paper held level, I applied to the lower side of it, just beneath the heap, the pole of a 

 vigorous load-stone, whose emissions diffusing themselves thro' the metal, manifestly alter'd 



