10 
Mr Brooke 07i Crystallisation. 
which he concludes, that simplicity of form may be influen- 
ced by art. He states, that mechanical mixtures^ if they re- 
main permanently suspended in the solution of a salt, will not 
produce any effect on its form ; but if an impalpable powder be 
suffered to occupy the lower part of the vessel, the forms of the 
crystals deposited in that, will be more regular and simple than 
of those which are deposited above it. And certain crystals of 
axinite, which are coated by chlorite ; the crystallised sandstone 
of Fontainbleau ; and the primary quaitz crystals, formed in 
an earthy ferruginous matrix, are adduced as natural illustra- 
tions of this fact. 
There does not, however, appear to be much constancy in the 
result of such experiments ; but, on the contrary, different sub- 
stances appear to possess peculiar habitudes in this respect, and 
sometimes to exhibit very discordant results. I have observed 
sulphate of potash deposit compound hi-pyramidal crystals in a 
stratum of impalpable powder at the bottom of a saucer, while 
the crystals which rested above this stratum were all simple. 
Chemical mixtures also influence the forms of crystals, and 
perhaps more frequently than any other cause, yet in a manner 
of which we believe at present no clear conception can be 
formed. Mr Brochant remarks, with great justice, that the na- 
ture of this influence is a highly important point in the history 
of crystallization, as it is probable that it very frequently go- 
verns the formation of natural crystals. Some chemical mix- 
tures may be said to influence even the primary forms of crys- 
tals, without apparently altering the nature of their chemical 
constitution, while others merely occasion changes in the modi- 
fications of those forms. 
Thus sulphate of nickel, if crystallised from an excess of acid, 
2 l square prism as the primary form; but if the square 
prisms thus obtained be dissolved in water, and re-crystallised, 
rhombic prisms are produced, without any apparent atomic dif- 
ference in the proportions of their respective elements. 
The difference between the forms of carbonate of lime and ar- 
ragonite, which are probably similar chemical compounds, may 
have resulted from some interference analogous to this ; and we 
may perhaps refer to the same cause the difference of figure be- 
tween the common and the white iron-pyrites. The influence 
