384 Proceedings of Philosophical Societies. [May, 



Having remarked that the secondary forms of a substance are 

 most commonly the same in any one mine or place where it is 

 found associated with other minerals under similar circumstances; 

 he judged that the secondary forms arise from the medium in 

 which the ciystallization takes place. 



It has been known for a long time, from the experiments of 

 Rome de T.ille, and those of Fourcroy and M. Vauquelin, that 

 the presence of urea occasions common salt to take an octahe- 

 dral form, although in pure water it crystallizes in cubes, similar 

 to its constituent molecules. The same substance produces an 

 opposite effect upon muriate of ammonia, which ciystallizes in 

 octahedrons in pure water ; while urea causes it to crystallize in 

 cubes. 



A very slight excess or deficiency of base in alum causes it to 

 assume either cubical or octahedral secondary forms ; and these 

 forms are so truly secondary that an octahedral crystal of alum 

 immerged in a solution which is richer in respect to its basis, 

 becomes enveloped with crystalline layers, which cause it to 

 assume at length the form of a cube. 



Setting out from these facts, M. Beudant treats the question 

 at full length, and has submitted the crystallization of salts to 

 experiments made under every circumstance that he believed 

 capable of exerting any influence upon it ; namely, 



1. General and external circumstances; such as heat, weight 

 of the atmosphere, greater or less rapidity of evaporation, bulk 

 ef the solution, form of vessels, &c. 



2. Mechanical mixtures which foul the solution, whether 

 simply suspended in the state of an incoherent precipitate, or in 

 that of a gelatinous deposit. 



3. What he denominates chemical mixtures existing in one 

 and the same solution. 



4. Lastly, variations in the proportion of the constituent prin- 

 ciples of the crystallized substance. 



The circumstances of the first kind do not exercise any other 

 action than what regards the size and perfection of the crystals. 

 The case is even the same, as to any small portion of matter which 

 remains in permanent suspension in the liquid ; but this cannot 

 be said in respect to precipitates and chemical mixtures. 



Crystals formed in the midst of an incoherent precipitate, 

 deposited like mud at the bottom of a liquid, always take up a 

 more or less considerable portion of the molecules of this precipi- 

 tate, and by this admixture they usually lose all those small addi- 

 tional facets which would otherwise modify their predominant 

 form. Thus the crystalliue form acquires greater simplicity when 

 it should apparently become more complicated ; at the same time 

 the substances which would otherwise have yielded simple crys- 

 tals still continue to yield them, and they do not receive any 

 modification. 



In a gelatinous deposit, crystals are rarely found in groups. 



