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MICROSCOPICAL JOURNAL. 



155 



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 fcrrocvanide, or prussi- 

 ate of potaL-h, 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 a 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 successfully employed with many 

 other salts. 'J'he 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 bal- 

 sam, and was annoyed to find all 

 spoilt in one afternoon. 



If this salt is allowed to crystallize 

 gradually, from a cold saturated solu- 

 tion in water, the crystals will be large ; 

 but if a nearly saturated solution be 

 made in pure alcohol, it will crystal- 

 lize 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 commercially 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 ap- 

 pearance 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, 

 whicii when quite dry, which is abso- 

 lutely necessary, are best mounted in 

 balsam and benzole. 



Ammonium oxalate also forms a 

 beautiful object, if an aqueous solu- 

 tion is evaporated upon a slide, and 

 mounted in pure balsam ; but more 

 beautiful than this, is the ammonium 

 oxalurate, which is deservedly pop- 

 ular. 



Of this latter salt make a hot satu- 

 rated solution in water ; place a little 

 upon a cold slip and warm very gently, 

 when circles composed 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 microcrystal is 

 increased if a fine point of wood is 

 held in the centre of the fluid, when 

 the needle-shaped crystals will radiate 

 from this artificial centre. 



If instead of this hot saturated so- 

 lution we take a cold but not quite 

 saturated solution in water, and evap- 

 orated still more slowly upon a cold 

 slide, the crystals will assume more 

 distinctly the circular or wheel char- 

 acter previously described ; and they 

 are so delicate that when mounted in 

 pure balsam, they become almost un- 

 distinguishable unless examined by 

 polarized light. 



Before leaving this group I must re- 

 fer 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 



