MICRO-CRYSTALLIZATION. 29 

lamp ; and at the same moment it will frequently be found advan- 
tageous to blow upon the surface of the slide. I cannot offer any 
reason for this why it does good, but it is often to be noticed that 
these little dodges improve materially the character of a slide. 
Potassium Ferrocyanide, or Prussiate of Potash, can be made to 
assume very beautiful dendritic forms, but the plan found to 
answer best with this is to make a hot, nearly saturated, solution in 
water, and then placing a drop of the solution on warm slide, jerk 
off with a rapid movement of the hand all surplus liquor. The 
jerk, which leaves but a thin film upon the slide, also very quickly 
causes the cooling of the slide and the instant crystallization of 
the salt. It may be suggested that the same process may be sucess- 
fully employed with many other salts. The Potassium Chlorate is 
said to be a success by this method, as also is Barium Chloride. 
Biborate of Soda, or Borax, is not a difficult slide to produce, 
but when formed and mounted safely in Balsam and Benzole, it 
must be allowed to dry naturally, as the application of even very 
moderate heat will cause the utter destruction of every crystal. 
This I learned by sad experience, for some time since having 
mounted several dozens of this slide I at once placed them above 
my hob to harden the Balsam, and was annoyed to find all spoilt 
in one afternoon. 
If this salt is allowed to crystallize gradually, from a cold satu- 
rated solution in water, the crystals will be large ; but if a nearly 
saturated solution be made in pure alcohol, it will crystallize 
almost immediately it is placed upon the cold slide, but the 
crystals will be much smaller than those from the aqueous 
solution. 
Turning now for a short time to the Salts of Ammonium, I will 
direct your attention to the Ammonium Chloride, known commer- 
cially as Sal Ammoniac. This requires only a moderately weak 
solution in cold water, of which a drop is to be placed upon the 
slide and then drained off, leaving but little upon the surface. 
This must then be gently warmed by holding over a spirit lamp, 
at the same time cooling from above by blowing upon the slide as 
described before. 
Ammonium Bitartrate differs much from the last described both 
in appearance and in mode of production. It requires a very 
strong solution in hot water, of which solution a drop should be 
placed upon a warm slide, and if allowed to cool gradually small 
and distinct crystals will be formed, which when quite dry, which 
is absolutely necessary, are best mounted in balsam and benzole. 
«Ammonium Oxalate also forms a beautiful object, if an aqueous 
solution is evaporated upon a slide, and mounted in pure balsam ; 
but more beautiful than this is the Ammonium Oxalurate, which 1s 
deservedly popular. 

