270 
Transactions of the Society. 
VII. — A few Notes on Micro-Crystallography . 
By T. Charters White, M.R.C.S., L.D.S., F.R.M.S. 
( Bead 1 6th February , 1898.) 
Every member of a corporate body ought, in my humble opinion, to 
do something to further the objects for which that body is incorporated. 
I regret I can do but little in forwarding the progress of our Society, 
but <l I do my diligence to give gladly of that little,” in offering the 
small contribution of this evening. I must ask you to consider it 
simply as a casual communication, being as it is a brief record of a 
few experiments made with a view of producing pretty polarising 
objects, which I thought might interest my fellow members as a 
relief from more arduous investigations. 
When I use the term “ pretty ” as applied to these objects, I do 
so because I think at present that is the only attribute to which they 
are entitled ; but as great events often spring from small beginnings, 
so these experiments and illustrations may be utilised by others, and 
may prove the threshold of something more important as years roll on. 
The subject of crystallisation in its larger aspect is outside the 
sphere of microscopy ; but we are all familiar with the fact that a hot 
and saturated solution of some saline substance will deposit crystals 
on cooling, and that these crystals assume definite shapes peculiar to 
the salts dissolved. Watts in his Manual of Chemistry says, “ Almost 
every substance, simple or compound, capable of existing in the solid 
state, assumes under favourable circumstances a distinct geometrical 
figure, usually bounded by plane surfaces and having angles of con- 
stant value.” However correct this definition may be when applied 
to the crystallographic systems under which we find grouped the 
cubic form, the rhombohedra, and so forth, we must remember that 
in a solution the crystals are free to build up the system to which 
they rightly belong ; but in the method of crystallisation it is my 
privilege to call your attention to this evening, the forces of crystal- 
lisation are locked up in a hard and attenuated film, instead of being 
free to act as in a solution, and are only set free when the hygroscopic 
character of the surrounding atmosphere gradually liberates them. 
Hence the presence of much or little moisture will modify the forms 
produced ; as will also the character of the surrounding air, whether 
it be gaseous, alcoholic, or ammoniacal ; so will varying degrees of 
temperature ; and it will be an almost endless source of entertainment 
to try and produce varying conditions under which micro-crystals 
may be formed. But it will be found, after you have tried the 
three or four chemicals that I shall introduce to your notice, that 
you will have come to the end of your tether in the production of 
pretty crystals ; and it will save a great deal of time and trouble in 
making experiments, if I name those I have found successful, and 
