1891.] MICKOSCOPICAL JOURNAL. 107 



water in the pocket for an hour or two the dormant stream will wake 

 up to activity. 



It is often desirable in a mixed company to exhibit something pretty 

 rather than scientific. Having carefully focused a plain slide on the 

 stage of a mici'oscope let some one look through the eye-piece while 

 a drop of spirit of camphor is run on the slide. At first nothing is seen. 

 The spirit of camphor when first put on does not shov/ anything under 

 a spot lens or a paraboloid, but as soon as the evaporation of the spirit 

 causes the crystals to form they show up suddenly as a silvery efflores- 

 cence, which is both startling and beautiful. Nothing exhibits the ca- 

 pabilities of a good spot lens so well as this. Another experiment may 

 also be exhibited, as an opaque object lighted by the bull's-eye condens- 

 ing lens : place a drop of a solution of nitrate of silver in distilled water 

 on the slide, and nicking a very small scrap of a bronze coin oft' with 

 vour penknife, wet this scrap with distilled water, and place it in the 

 centre of the drop of silver solution. Immediately bright leaves of 

 glistening silver dart out from the metal, fornnng not only an attractive 

 object at the time, but one which may be mounted permanently. Crys- 

 tallization is always watched Vk'ith great interest, and is one of those 

 subjects which the student should familiarize himself with, so that he 

 may readily recognize the crystals of the various salts he may meet with 

 in his investigation, always bearing in mind that outward influences 

 modify shape in crystals as well as in animal life. Let him crystallize 

 bichromate of potash from a plain aqueous solution on a slide and com- 

 pare it with another having a little gum arable dissolved in it, and he 

 might mistake them for difterent salts. Let him modify the tempera- 

 ture, and he will detect a slight change ; but the most amazing changes 

 may be produced from a solution of hippuric acid in absolute alcohol ; 

 such changes that every slide showing a difterent aspect might be mis- 

 taken for a separate salt. Let the solution be warm, the dipping-rod 

 and the slide warm. As soon as the solution touches the slide it dif- 

 fuses itself and becomes like a transparent film. Soon the moisture 

 of the atmosphere begins to liberate the force which keeps back crys- 

 Lallization, and the film is studded with round centres of commencing 

 crystals. If now the slide be alternately warmed and cooled, a series of 

 concentric circles will be formed round the primary dot ; these increase 

 until contact with adjoining systems meet, and crystallization ceases ; 

 if instead of letting these circles increase, a sudden accession of mois- 

 ture be supplied, as by quickly breathing on the slide, then rays of crys- 

 tals start out from the edges of the circles, and a series of stars of varying 

 size are formed. If this is done, accompanied by a moderately high 

 temperature, the crystals form spirally ; a higher temperature melts them, 

 and their power of recrystallizing is destroyed. Crystallization, under 

 the influence of difterent gases, considerably modifies the result, while 

 crystallization from an alcoholic solution in which arabin takes part 

 will make the cr3'stals take a form totall}- unlike any other, and capable 

 of being examined as an opaque object. When it may be thought de- 

 sirable to mount any of these crystals, castor oil forms a good preserva- 

 tive ; balsam causes them to break up in a most unsightly manner. 



There is, perhaps, no movement observable under the microscope to 

 which the student should direct his attention more important than those 

 movements of minute particles of matter which are known as Brownian 



