130 Proceedings of the lioyal Society 
but not with all that were tried. Among the successful instances 
may be mentioned sulphate of copper, binacetate of lead, muriate 
of morphia, and other similar salts, which afford beautiful crystals, 
and are very easily manipulated. The method of proceeding is 
similar in all ; for instance, to a solution of sulphate of copper, 
let gum arabic or common dextrine be slowly added until it pours 
oily, then tried, and more gum or more salt added until the result 
is satisfactory. No precise general rule can be given, as each salt 
will be found different in the quantity of gum required. The 
solution is then to be spread thinly over a plate of glass, and dried 
rapidly before a clear fire, or, if the plate is small, it may be dried 
over a gas-burner. Upon cooling, the plate, if left to itself, soon 
shows numerous small specks, which will gradually develope them- 
selves into circular crystals. The process is hastened and a better 
result obtained by breathing on the plate, when, after a short time, 
they may be observed to start out very beautifully. These crystals 
while growing are extremely sensitive, any variation in the moisture 
applied for their formation resulting in the production of rings. 
The growth of the crystals once begun is extremely regular. 
When the centre is of inappreciable size they are circular, and they 
proceed onwards in that form until stopped by other crystals, or 
until the whole vacant space is oceupied by them. Should the 
origin have a definite shape, then that is carried on by the crystals 
arranging themselves always perpendicularly to the outline of that 
origin, while, should it be a straight line, they form beautiful 
fringes perpendicular to the line, and terminate at each extremity 
in semicircles. 
The centres round which the crystals arrange themselves maybe 
either some foreign body in the film, or be determined by some 
molecular arrangement of the salt at a particular point in the film. 
They seem to originate spontaneously, and subject to no apparent 
rule, for foreign particles, and even minute crystals, will not always 
determine centres ; in fact a film may be full of little crystals of the 
salt, and to very few of them can the circular arrangement be traced. 
The crystals present themselves under two aspects in all the 
salts that I have examined, — a true and an abnormal form. I 
designate the true form as that in which the crystallisation proceeds 
by the formation of spicular crystals radiating from a centre and 
