457 



of the well-known fact, that in bodies which crystallize from a state 

 of igneous fusion, the most perfect crystalline state is produced 

 when the longest time intervenes between the commencement of 

 solidification (now using that term in its ordinary sense) and the 

 complete cooling of the melted mass. The cases cited from Mr. 

 Faraday at the beginning of this paper, of the growth of crystals 

 (including those of ice in ice-cold water) in solutions, all have their 

 exact parallels in the accretion of crystals in cooling melted glass. 

 " Crystals of ice," Mr. Faraday observes, " which could not be colder 

 than the surrounding fluid, exhibited the phenomena of regelation " 

 that is, of incorporation into one " when purposely brought in 

 contact with each other." The same thing happens with melted 

 glass slowly cooling, in which crystalline spherules, often forming 

 spontaneously and independently, continue to form and to increase, 

 even after the glass has become solid as such, by the operation of a 

 principle in this view analogous to regelation, until the entire mass 

 has become crystalline*. 



3. No crystalline body has been longer or more extensively subject 

 to human observation, than crystallized water, or ice. Its natural 

 history and properties, as science has advanced, have been investi- 

 gated with increasing generality and precision ; and they have finally 

 become objects of that systematic and exact research which charac- 

 terizes the present era of physical inquiry, as is evinced by the dis- 



* If we should prefer to adopt Mr. Maskelyne's suggestion in a formal manner, 

 and regard glass as resembling a solution about to crystallize, its analogue, 

 agreeably to the preceding views, will be a saturated solution of a salt in hot 

 water, allowed to cool undisturbed, and remaining fluid, until its cohesion is 

 affected, when its temperature rises, and the salt crystallizes. Specimens of glass 

 are common which have the aspect and distribution of parts of a crystallized salt 

 in the mother-liquor ; opake crystallized spherules appearing in the midst of a 

 transparent mass. To these correspond, among natural glasses, pitchstone and 

 many examples of porphyritic obsidian, consisting of a vitreous base in which 

 crystals have been formed and are imbedded. 



But at the same time the view I have taken of the subject, and Mr. Maskelyne's, 

 may be equally tenable ; for the state of water remaining liquid at temperatures 

 below 32, and that of saline solutions remaining un crystallized at temperatures 

 below those of solidification, are evidently closely analogous. 



Should I return to this subject, I shall refer to my friend Mr. Sorby's observa- 

 tions on the nature of glass, which I had not read when these notes were com- 

 municated to the Royal Society, but which are in entire agreement with the views 

 I have suggested. See Quart. Journ. of Geol. Soc. vol. xiv. p. 465. 



