MEMORANDA. 2038 
Herewith I send you four specimens of crystals photo- 
graphed by polarized light.* The crystals of “tartrate of 
soda” is magnified forty diameters, and may be produced by 
neutralizing a strong solution of carbonate of soda with tar- 
taric acid. A drop of this is then spread upon the slide, and 
the water evaporated by heat, when the salt will remain in a 
viscid state. The slide must then be laid in a dry place, free 
from dust, and in a short time will be covered with crystals, 
where the formless salt before lay. This will sometimes be 
found to be accomplished in a day, but at other times a week 
or two must elapse before the same result takes place, accord- 
ing to the warmth and dryness of the atmosphere. The 
crystal of sulphate of copper and magnesia is also magnified 
forty diameters, and was obtained by the method I havee lse- 
where described. The crystal of santonin is also magnified 
forty diameters, but this salt requires a very different treat- 
meut to procure good crystals. A small portion is placed 
upon the centre of the glass slide, and this is then laid upon 
a metal plate, underneath which a spirit-lamp must be kept 
burning, in order that the temperature may not be lower 
than 350° Fahr., but if it is raised above 400° the crystals 
will turn brown. In a short time the santonin becomes 
fused, and with a hot needle should be thinly and evenly 
spread upon the surface of the slide, which should then be 
allowed to cool slowly. When the surface has become 
“fixed,” a fine-pointed needle may be employed to pierce it 
here and there, when the crystals will spread from these cen- 
tres and cover the plate. Castor oil must be used in mount- 
ing them, as balsam is apt to dissolve them when the tempe- 
rature is raised even im a slight degree. The difficulty in 
preparing this salt is to know the precise moment at which 
to start the formation of the crystals, as those produced be- 
fore and after this moment are devoid of that feathery deli- 
cacy which is their chief beauty. The crystals of tartar 
emetic are magnified fifty diameters, and were produced in 
the ordinary way. I have taken the trouble to describe the 
production of these crystals, because I am not aware that 
some of them have been before noticed as microscopic objects, 
and they are well worthy of a place in the cabinet.—THomas 
DaviEs. 
* We had intended to give copies of the beautiful photographs sent by 
Mr. Davies, and which fully bear out his remarks, but find that the expense 
of this mode of illustration would be far too great, and are, therefore, 
obliged to forego what would otherwise have rendered his communication 
far more complete.—[Ebs. ] 
