164 



HARDWICKE'S SCIENCE-GOSSIP. 



[July 1, 18GS. 



MICROSCOPY. 



The Silver. Tree.— Readers of Science-Gossip 

 who have tried the experiment suggested by 

 R. H. N. B. will perhaps be glad to know that the 

 Silver Tree is far more beautiful as a microscopic 

 object than the one which was described in the June 

 number. A few drops of a solution of nitrate of silver 

 in water should be placed in a glass cell or upon a 

 hollowed glass slide, and on dropping a small piece 

 of clean copper wire into it, a growth of exquisitely 

 beautiful fern-like crystals in burnished silver will 

 immediately commence, starting from the copper and 

 continuing to spread in all directions until the whole 

 of the silver in the solution is deposited in a metallic 

 form. A still better way of exhibiting the experi- 

 ment is to put some of the solution into a copper 

 cell, such as I am in the habit of using for dry mount- 

 ing ; — the silver ferns will then start simultaneously 

 from all parts of the iuner circumference of the cell, 

 and will grow out evenly towards the centre. In 

 examining them, both reflected and transmitted 

 light should be employed"; the former diffused 

 through ground or opal glass, and just strong 

 enough to show clearly the outlines of the crystals, 

 the latter as intense as possible, to show the beau- 

 tiful effects produced by their highly reflective 

 surfaces. Slides so prepared should be placed 

 under the microscope immediately, since the newly- 

 formed compound, nitrate of copper, speedily makes 

 its appearance, and as the water evaporates deposits 

 itself in a crystalline form upon the metallic silver. 



The foregoing methods are those which are the 

 most readily adopted ; but if, instead of decom- 

 posing the solution by the introduction of the 

 copper, a weak galvanic current is employed, the 

 experiment becomes one of greatly increased in- 

 terest. The solution should in this case be placed 

 either in a glass cell or in a zoophyte-trough, and 

 the ends of two fine platinum wires, carefully insu- 

 lated from the stage, should be immersed in it. 

 When the other ends of these wires are connected 

 with the poles of the battery, a beautiful growth of 

 fern-like forms in pure silver commences in the 

 fluid upon each wire, and proceeds with a rapidity 

 dependent upon the strength of the current and 

 that of the solution. It is very curious to notice 

 that the ferns proceeding from the two wires are of 

 different species, one sort being identical with 

 those produced by the copper, whilst the others 

 closely resemble in shape those which are precipi- 

 tated in a solution of lead by a piece of zinc ; and 

 when it is remembered that in nature silver is 

 almost always found in combination with lead, this 

 similarity in the form of crystallization seems worthy 

 of remark. If during the formation of the crystals 

 the current is broken, their growth is instantly 

 arrested ; and if it be then reversed, the beautiful 



silver fronds begin at once to ungrow ; the leaf-like 

 forms which, as you watched them, were so wonder- 

 fully put forth from their graceful stems, now 

 mysteriously retire into those stems again ; these 

 too, in their turn, likewise disappear until the 

 last vestige is dissolved, when a new growth in- 

 stantly commences — the different forms this time 

 making their appearance upon the opposite wires. 

 The experiment is one of great beauty, especially 

 as seen by a good light with a low power and a 

 binocular ; but it obviously requires some care and 

 much steadiness to perform successfully. A very 

 weak current only is required ; indeed, with a 3-inch 

 bichromate battery I find it necessary only to allow 

 the extremities of the plates to touch the exciting 

 fluid, otherwise the water itself is decomposed, and 

 the crystals are detached from the wires by bubbles 

 of the disengaged gases. When thoroughly washed, 

 the crystals of silver may be mounted dry, and if 

 the ail- is excluded from them, they will retain their 

 brilliancy untarnished for many years. I may also 

 add that in bringing electricity upon the stage of 

 the microscope, two short dipping-tubes, mounted 

 in the same way as the stage forceps, make excel- 

 lent insulators, and at the same time will hold and 

 maintain the wires in any required position ; and 

 a commutator included in the circuit places the 

 current under complete control, and enables the 

 operator to break or reverse it at will. — R. T. Lewis. 



CrLETOsriRA Mulleri. — In Mr. Tatem's drawing 

 of Ch&tospira Mulleri (Science-Gossip, No. 42), I 

 was glad to recognize an infusorian, a solitary speci- 

 men of which I met with last summer on the stem 

 of a water-crowfoot from this neighbourhood. At 

 the time, I sent a drawing of it to a friend who had 

 access to a number of books on the Infusoria, re- 

 questing him to identify it, but he did not succeed 

 in doing so. I would remark that in this specimen, 

 instead of the internal cellular tissue being broken 

 down to afford it protection, it had built a tube of 

 minute foreign particles, distributed over the surface 

 of an inner and semi-transparent cylindrical tube, 

 which ran for a little distanee in the direction of 

 the stem, and then turned almost at right-angles to 

 it. In the respect of its building a tube it would 

 resemble Cluetospira mucicola, but I am certain that 

 it was Chcetospira Mulleri from the fact of the spire 

 when protruded making a complete turn. It may 

 have been another species.—/. J. It., Weston-super- 

 Mare. 



Volvox globator.— In No. IS (June, 1SG6), 

 " J. S." speaks of finding that favorite microscopic 

 object Volvox globator on New Wandsworth 

 Common. I have found it this year in considerable 

 numbers— for the first time to my knowledge— in a 

 pond on Clapham Common, between the Windmill 

 Inn and the Cavendish Road. — M. E. H. 



