
30 THE NORTHERN MICROSCOPIST. 

Of this latter salt make a hot saturated solution in water; place 
a little upon cold slip and warm very gently, when circles com- 
posed of numerous needle-shaped crystals will commence at the 
edge of the drop of fluid and extend gradually towards the centre ; 
but the beauty of this micro-crystal is increased if a fine point of 
wood is held in the centre of the fluid, when the needle-shaped 
will radiate from this artificial centre. 
If instead of this hot saturated solution we take a cold but not 
quite saturated solution in water, and evaporated still more slowly 
upon.a cold slide, the crystals will assume more distinctly the cir- 
cular or wheel character previously described, and they are so 
delicate that when mounted in pure balsam, they become almost 
undistinguishable unless examined by polarised light. 
Before leaving this group I must refer to Ammonium Purpurate, 
frequently known as Murexide, a salt which is obtainable by the 
decomposition of Uric Acid, and is also found among urinary 
crystals in certain abnormal conditions of the secretive organs. 
A solution of this salt in water, upon evaporation, deposits the 
crystals in short and flattened four-sided prisms which show well 
by polarized light, and possess the analytic character referred to 
later. 
Besides the salts of Ammonia described may be enumerated the 
Borate, Muriate, Phosphate, Sulphate, Urate, and Platino-cyanide ; 
with these I have had but little experience. I may say, however, 
that of themselves, and in combination with other things, you will 
find them worthy objects of study or amusement. 
Turning now to Magnesium, we find that at present there are 
not so large a number of its salts that are recognised as micro- 
crystals ; but I must not omit from these the well-known Epsom 
Salts, or Magnesium Sulphate, which will produce, according to 
the mode of crystallization, very varying results. The most typical 
and one readily produced, is obtained from a saturated, or nearly 
saturated, aqueous solution, which, if allowed to crystalize very 
slowly on a slide, will result in much larger crystals than if the 
slide, with a drop of the solution, is very gently warmed. ‘These 
crystals, if mounted in pure balsam, will, with selenite, form a 
brilliant slide. 
Ammonia Phosphate of Magnesia, or as it is frequently called, 
Triple Phosphate, is like the Murexide referred to before, a pro- 
duction from urine, and occurs in many different forms, of which 
the one known as the prismatic form is the most frequent. This 
salt is not a commercial product, but it is said that the crystals 
may be obtained by allowing urine to decompose, or by diluting 
the secretion with water and adding in small quantities, dilute 
Ammonia solution. 
One other salt of Magnesia, the Platino-cyanide, must be re- 
