272 
Transactions of the Society. 
The third chemical I have experimented with is picric acid dis- 
solved in alcohol ; but this, while giving a variety of crystals that were 
rather peculiar, inasmuch as it gathered itself up in strange Japan- 
esque designs, induced me to believe that by combining with it a 
solution of hippuric acid I might produce a modification of this char- 
acter, and it produced a conformation quite different from any I had 
ever previously obtained. I will call your attention to it when thrown 
on the screen. There is no difficulty in obtaining the very curious 
crystals obtained by the use of bichromate of potassium in gum arabic. 
An aqueous solution of the bichromate is incorporated with some gum 
arabic, and this is placed upon the slide and warmed. As soon as a 
pellicle is seen to form around the margin of the drop of solution, it is 
put aside to cool, when crystallisation takes place, giving a peculiar 
twisted appearance to the crystals ; and these, taking up the bulk of the 
solution, leave a thin layer of fluid which results in a fine feathery 
spray in the interspaces left between the larger crystals. This salt is 
not capable of any great modification. 
I have photographed any particular form that struck me as being 
curious or out of the ordinary character, as I have experienced great 
difficulty in preserving the specimens when mounted. No difficulty 
is met with in mounting hippuric acid crystals if mounted as a dry 
object, but hydroquinone will not stand even that treatment, but 
breaks up under the cover-glass when shut in ; I have therefore gene- 
rally kept it on an open slide. Neither hippuric acid nor hydro- 
quinone bears mounting in any of the usual fluid media, or in balsam, 
as hydroquinone disappears altogether, and hippuric acid breaks up 
and becomes a disfiguration instead of a thing of beauty and a joy for 
ever. But these solutions may be made and will keep indefinitely, 
so that they are always ready for a demonstration, the hydroquinone 
crystals being very attractive for dark-ground illumination. 
