THE PRODROMUS 243 



formation of glass without the agency of fire, provided one 

 undertake a careful analysis of the rocks in whose cavities the 

 best crystals are formed. For it is certain that, just as a crystal 

 has formed from a fluid, so that same crystal can be dissolved into 

 a fluid, provided one know how to imitate the real menstruum ' of 

 Nature. And it is no disproof that certain solid bodies, when 

 once the dissolving fluid, or their menstruum, has been taken 

 away, can be no further disintegrated by the same or a similar 

 solvent; for this occurs in bodies from which the entire men- 

 struum is freed by the force of fire. But the crystal, and 

 all angular bodies which form in the midst of a solvent fluid, 

 or in the midst of a menstruum, can never come out so pure 

 but that some particles of the menstruum are left within the 

 particles of the angular body. And upon this fact depends the 

 main cause of variation whereby crystal differs from glass not 

 P 45. only in refraction but also in other properties, since in glass no 

 parts of the dissolving fluid are present, inasmuch as they have 

 been driven forth by the violence of fire. For the fluid, in 

 which the crystal is formed, bears the same relation to the 

 crystal that ordinary water bears to salts; this could easily be 

 proved by setting forth the characteristics which the formation 

 of salts holds in common with the formation of crystal. 



But since I should be wandering too far from my subject if I 

 should allude to all these things here, I shall mention but one 

 example, which seemed exceedingly beautiful to me. In various 

 places within the same stone the receding layers were filled with 

 crystals, of which some were watery, others very clear, some 

 white, many amethystine, mingled together without any blend- 

 ing of hues ; exactly as experiments with salts made here show 

 that vitriol and alum, dissolved in the same water, after a part 

 of the water has been taken up, have each formed by them- 

 selves without any mixing of parts. 



1 The term 7nenstruum, used by Steno, was commonly employed by the alchemists and 

 physicists to denote a solvent fluid. Compare Littrd, Dictionaire de la Langue Fran^aise, s.v. 

 menstrue: "Terme de chimie. Liqueur propre \ dissoudre les corps solides. L'eau regale 

 est le menstrue de I'or {aqua regia). On dit aujourd'hui de prdf^rence dissolvant." 



Excellent examples of this usage can be found in Robert Boyle's works ; compare, e.g., 

 Experiments and Observations upon Colours (edition of Shaw, Vol. II, p. 96) : " That gold, 

 dissolv'd in Aqua regia, communicates its own colour to the menstruum, is a common obser- 

 vation ; but the solutions of mercury, in Aqua fortis, are not generally observ'd to give any 

 notable tincture to the menstruum. See also New Oxford Dictionary, s.v. menstruum. 



