A few Notes on Micro-Crystallography. By T. C. White. 271 
that have been so much pleasure to me. They are hippuric acid, 
hydroquinone, picric acid alone and in combination with hippuric 
acid, and an aqueous solution of bichromate of potassium crystallised 
in a tolerably thick emulsion of gum arabic. This is the only aqueous 
solution ; the other solvents have been methylated spirit, acetone, and 
absolute alcohol, taking these three solvents as types of the greatest 
volatility, because in making these crystals it is necessary that the 
solvent should evaporate very quickly, or else the crystals will assume 
to a great extent their natural forms. It is desirable to make satu- 
rated or even super-saturated solutions of the three chemicals named, 
as the colours produced under polarised light are of a deeper and 
richer character than they are if made from weaker solutions. 
It will be found that of these three, hippuric acid is the most 
manageable, allowing of more time being taken in modifying the 
forming crystals. To produce these varying forms it is advisable to 
have the microscopical slide slightly warmed so that it is not unbear- 
ably hot if placed on the back of the hand, as the evaporation of the 
spirit is accelerated thereby, and as it spreads it leaves a clean thin 
film on the glass. Should there be moisture in the air of the room, 
a number of dots appear in this film, marking the commencing forma- 
tion of centres of crystallisation. If now the slip is alternately warmed 
and cooled, these centres will be seen surrounded by concentric rings 
formed by the alternations of temperature to which they have been 
subjected. If they are blown upon while under the Microscope, an 
advancing wave of a fringe-like character may be watched as it 
increases the diameter of the growing crystal ; if it now be sharply 
breathed upon, a star-like radiation will surround each centre, and 
the crystallisation will be completed. Now this is the physical 
history of one such formation, but the modifications in the character 
of these crystals may be made very numerous by changing the con- 
ditions under which they are formed. I shall have an opportunity 
afforded me of showing you by means of the lantern some of the 
forms assumed by this salt. 
Hydroquinone crystallises fairly well out of an alcoholic solution, 
but its best effects are produced from a solution in acetone ( = 
pyro-acetic spirit, C 3 H 3 0) ; this quickly dissolves the hydroquinone, 
especially if it is boiled ; and as it boils at 132° F., this method ot 
getting a saturated solution can be readily accomplished ; but, owing to 
the extremely rapid evaporation, it is not possible to modify the re- 
sulting crystalline forms ; but these are so beautiful that one does not 
seek to modify them to any great extent. Unlike hippuric acid, 
which crystallises in round characters, hydroquinone resolves itself 
into very decided curvilinear forms, whose only chance of modification 
arises from the degree of warmth to which you subject the slide before 
putting on the solution ; but with these modifications most charming 
designs can be obtained. These salts are generally employed in single 
solutions unmixed with any other salt. 
1898 
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