Mahch 1, 1887.] 



♦ KNOWLEDGE ♦ 



107 



PLEASANT HOURS WITH THE MICROSCOPE. 



By Henry J. Slack, F.G.S., F.E.M.S. 



EW spectacles are more beautiful and few 

 more wonderful than to see the formation 

 of crystals either on the stage of the 

 microscope or with suitable materials on 

 a magic-lantern slide. In a former article 

 the exhibition of a silver tree in its rapid 

 process of development was especiall}- re- 

 commended. In this case a drop of solu- 

 tion of silver nitrate is placed on a slide, with the stage 

 of the instrument horizontal. This plan — except when 

 Stephenson's binocular is employed — is a little awkward 

 with a tall microscope, as the tube is u|)right, but no other 

 aiTangement answei'S so well. A very small bit of copper, 

 cut from the top of a wire, is put into the solution drop, 

 after the latter has been focussed with an inch power. The 

 chemical action is very rapid if the particle of copper is 

 quite clear. Instantly a tree-like form springs forth, and 

 the branchlets extend in all directions as if a living force 

 impelled their gi-owth. Each twig of the silver tree is seen 

 to be made up of small crystalline particles, but the still 

 minuter particles composing them are not discerned. The 

 little silver crystals that unite to imitate the pattern of a 

 plant are too thick to be translucent, as silver leaf is, and 

 should be viewed under strong reflected light. 



If we take another substance — say, tartaric acid, which 

 crystallises very quickly from its concentrated solution — we 

 see much the same process as with the silver solution, but 

 the pattern is quite changed. Silver crystallises in octo- 

 hedrons, and the native crystals often aggregate into thread- 

 like and arborescent patterns, as in the microscope experi- 

 ment. Tartaric acid crystallises in complicated patterns, 

 which appear to be modification.? of a rhombic prism. 

 The tendency to complication makes it especially beautiful 

 for microscopic exhibition, and its development should 

 be watched under polarised light, which gives splendid 

 colours. Even if the process is watched under a 

 high power, the eye gains no vision of minute particles 

 coalescing to make up any portion of the pattern. Each 

 pattern visibly grows, and very fast, if the experiment is 

 performed in a warm room and evaporation is rapid, but 

 exactly how it grows is not made clear. 



If, therefore, we want to see a little more of the way in 

 which non-crystalline particles adhere to form crystals, we 

 must select some substance that can be made to work in a 

 manner and at a pace adapted to satisfy our curiosity. 

 This was accomplished a few years ago by a German phi- 

 losopher named Vogelsang. He thinned a little Canada 

 balsam with bisulphide of carbon, which readily dissolves it, 

 and in another small vessel dissolved a little bit of sulphur 

 in the same fluid. Two half-dram tube bottles answer very 

 well for the experiment. Two or three drops of the balsam 

 in the state in which it is usually supplied, and about as 

 much carbon bisulphide, will probably answer, but the 

 amount of thinning the balsam requires must be learnt by 

 a little practice ; and it should be remembered that in 

 thinning Canada balsam with any of its best solvents, the 

 transition from being too thick to becoming too thin is very 

 sudden after a certain point of dilution has been reached. 

 A bit of brimstone the size of a peppercorn crushed into 

 powder dissolves quickly in three or four times its bulk of 

 the bisulphide. 



Having prepared the two solutions, take a small glass rod, 

 or, what will answer, two lucifer-match sticks. Use one for 

 the balsam and one for the sulphur solution, or if the glass 

 rod is employed clean it each time with a little benzine and 

 a rag, so as not to get any sulphur into the balsam bottle. 



Put a small drop of the balsam on to a glass slide with the 

 microscope stage horizontal, that it may rest quite still. 

 Then, having focussed it with an inch or half-inch power, 

 add to the balsam drop one of the sulphur .solution. Bi- 

 sulphide of carbon evaporates very quickly, and the crystal- 

 lisation process begins at once. Now, if the balsam is of 

 the right stickiness, it retards this process suiEciently to 

 enable its stages to be seen. First comes the formation of a 

 swarm of what Vogelsang calls glohuUtes : tiny spherules, 

 such as are formed by precipitation of a colloid substance, 

 like mastic, when its alcoholic solution is thrown into 

 water. These globulites are not like solid particles of the 

 sulphur- ; they are plastic bodies, and build up crystalline 

 forms of exqui-site beauty, which should be viewed in various 

 ways — with transmitted, and reflected, and with polarised 

 light. The sulphur precipitated in this way acts upon 

 polarised light in proportion to its advance in crystalline 

 development. The globulites that have not begun to make 

 crystals do not show any colours, while the most complete 

 crystals are in a brilliant blaze of prismatic hues. 



These experiments of Vogelsang are scarcely known in 

 this country except to the few students of micropetrology, 

 although making sulphur crystals for the polariscope has 

 not been uncommon. It is probable, as Vogelsang con- 

 sidered, that when the chemical molecules of any substance 

 are precipitated from solution and form cry.stals, the first 

 stage of the process is their aggregation into globulites, 

 which in many cases require high powers of the microscope 

 to discern, and in others escape vision, either from the 

 rapidity of the process or from their optical properties. 



In forming Mas-Schulze's artificial diatoms by a method 

 explained in a former paper, we obtain globulites of silica 

 which have enough plasticity to unite in vesicles like very 

 thin glass bulbs. The process consists in putting a spoonful 

 of powdered fluor spar, and one of fine silicious sand or 

 powdered glass, into a common G or 8-oz. wide-mouthed 

 bottle, pouring sulphuric acid over it, and lightly stopping 

 the mouth with a loose flock of cotton wool saturated with 

 water. It is well to let the apparatus rest for four-and- 

 twenty hours, and then delicately remove the vesicles from 

 the cotton threads, and wash all the acid out of them. 

 They are then fit for the microscope. 



The writer many years ago obtained exceedingly fine 

 silica films by passing silicic fluoride gas through glycerine 

 four parts to water one part. Here the adhesion of the 

 globulites is feeble, and the slightest touch broke the films. 

 Lately another experiment was tried, with interesting 

 results. 



If the silicic fluoride gas is given off pretty quickly, 

 which it will be if the bottle containing the preparation is 

 kept warm in a water bath, the cotton is soon saturated 

 with the fluosilicic acid and its fibre encased in silica. The 

 cotton should be allowed to dry, and then small tufts lightly 

 taken up with forceps and calcined in the flame of a spirit 

 lamp in a platina capsule or spoon, or in a little Berlin 

 crucible that will stand the heat. The capsule is the best, 

 as the porcelain crucible will most likely be broken in inex- 

 perienced hands. For the water-bath mentioned above, 

 take a round enamelled iron soap-dLsh, worth about .sixpence. 

 If the cotton is well calcined, and moved carefully, the 

 result is to obtain a sort of artificial fossil cast of the fibres, 

 in the form of a number of hollow sOica tubes, each built of 

 innumerable globulites. They .should be viewed with both 

 transmitted and reflected light, and present very beautiful 

 as well as curious appearances. 



Instead of cotton fibre, pieces of wet sponge may be 

 employed and calcined as before. A platina capsule about 

 the diameter of a threepenny piece is big enough for these 

 experiments, and can be obtained of the chemical apparatus 



