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will disappear, and the plate may be re-crystallised, but not so well 
as at first, and not from the same centres. This is the case with 
the binacetate of lead. 
There is one remarkable form of these crystals which is of fre- 
quent occurrence, and which Sir D. Brewster seems to have observed 
only in mannite. The form looks very extraordinary, a properly 
prepared plate presenting the appearance of being covered with 
paraboloids. These are simply circular crystals which have been 
formed in the film while the general crystallisation has proceeded 
from one side, and is caused by the crystals overtaking one another 
in their onward progress in one direction. Taking this case in its 
simplest form by supposing equal rates of crystallisation proceeding 
from the edge of a plate and from a centre near the edge, their line 
of contact will necessarily be a parabola, for it is evident the 
edge is a directrix and the centre the focus of such a figure, but 
this will very seldom occur, since the growths are not only generally 
unequal, but also vary in themselves. 
The question may be put whether gum arabic is the substance 
best suited for these experiments. I have not tried many, and 
with those that I have experimented upon the results were not so 
satisfactory. That other agents may be employed, according to 
the salts or other substances to be treated, is certain ; for example, 
collodion, as may be seen in a coated photographic plate that has 
been allowed to dry after excitation in the nitrate of silver bath, 
when circular crystals of iodo-nitrate of silver arQ often produced. 
This salt, be it observed, although produced through the agency 
of water, being at once decomposed by that element. 
Having confined my observations mainly to a very small number 
of salts, it would be premature to offer any general conclusion on the 
structure of their circular crystals or on their optical properties ; be- 
sides, from the great interest Professor Tait has shown in the subject, 
I am in great hopes that he maybe induced to make some investiga- 
tion in it. I shall only mention one point that I have observed in 
the effect of some crystals on the black cross, something in their 
structure producing a more or less spiral arrangement of the arms 
in a horizontal direction, while on one occasion a vertical arrange- 
ment was observed, in which the arms seemed to be raised one 
above the other like four quadrant steps. This effect I have only 
