82 



♦ KNOWLEDGE ♦ 



[Feb. 9, 1883. 



ground, so as to conceal the place where the precious 

 objects arc laid. In spite of all her precautions, however, 

 groat (juantities are found and eaten ; for the so-called 

 plovers' eggs of London game-dealers belong by just right 

 to the lapwing alone. After the young are hatched, the 

 peewits become more nocturnal in their liabits, and also 

 congregate in large flocks, instead of being seen in pairs, as 

 during the earlier part of the year. This is a common trick 

 with the plover trttjc, and it doubtless serves a good pur- 

 pose in the economy of the species. While nesting goes on, 

 in the flu.sh season, concealment and solitary habits are most 

 important ; in autumn and winter, the dearth of food 

 compels all alike to flock together towards the best and 

 richest low-lying feeding places. 



PLEASANT HOURS WITH THE 

 MICROSCOPE. 



By Henry J. Slack, F.G.S., F.RM.S. 



FEW English microscopists seem acquainted with Max 

 Schulze's artificial diatoms, and still fewer have niade 

 them. The process is very easy, and not only supplies 

 beautiful and curious objects, but also illustrates some pro- 

 perties of silica which are of importance in the organic 

 world. Take a wide-mouthed bottle that holds six or eight 

 ounces. MLx together a teaspoonful of powdered fluor spar, 

 and rather less of powdered glass or sharp, clean sand. Put 

 the mixture into a bottle, and pour upon it enough sul- 

 phuric acid to give it a thorough wetting. Have ready a 

 tuft of cotton wool, as loose as the fibres can be kept, after 

 dipping them in water. Place this moist tuft in the mouth 

 of the bottle, spreading it out so that it cannot tumble in ; 

 then put a paper cap over it to check evaporation, and 

 leave it for some hours, or till the next day. The acid decom- 

 poses the fluor spar and causes it to evolve fluoric acid, which 

 seizes some of the silica of the glass and carries it oflT up 

 to the wet cotton. Here, again, a chemical action takes 

 place, and part of the silica is deposited like hoar-frost 

 upon the cotton. This deposit should be scraped ofi" as 

 gently as possible with a little piece of stick — say a lucifer- 

 match, split with a penknife to give it a blunt edge. Let 

 it fall into a watch-glass, or any small vessel of glass or 

 china, and pour water very softly upon it, letting it 

 run ofl" quickly. This washes a corrosi\e acid out, 

 and leaves pure hydrate of silica behind in a variety 

 of curious shapes. Some will be nearly globular, others 

 like Amaltha'a's horn, others crooked tubes, etc. The largest 

 should be examined as opaque objects, and will be found 

 like strange glass vessels beautifully beaded. The finer 

 ones should be gently crushed and mounted as transparent 

 objects dry, and also in Canada balsam. They will most 

 likely exhibit a considerable number, perhaps a dozen or 

 more, of diatom patterns. Some will be finely and 

 uniformly beaded, like I'leurosigma angulatum. Others, 

 coarsely beaded, more like Triceraliuw. Some are likely 

 to be so fine and delicate that their br-ading requires a 

 magnification of 1,000 diameters or more to show it well. 

 Besides plain-beaded patterns, the writer has usually ol>- 

 tained compound patterns, such as large beads surrounded 

 by little ones. 



If a judicious selection is made, the fragments of the 

 little vesicles would be assuredly taken for portions of 

 well-known diatoms. 



It occurred to the writer that it would be interesting to 

 obtain the silica deposit in another way. Instead of 

 allowing the silicic-fluoride gas to come into contact with 



wet cotton, some of it was passed through a mixture of 

 four parts of glycerine and one of water. This is readily 

 managed by using a very small flask or a tube bottle to 

 contain the fluor spar, glass, and acid, and fitting to its 

 mouth a few inches of bent glass tube. A gentle heat 

 from a spirit-lamp causes the gas to be given off freely, 

 and by dipping the tube just under the glycerine and 

 water, which may be held in an egg-cup, silica films are 

 instantly formed. The experimenter must be on the watch 

 lest the tube gets stopped up with tlie silica deposit. 

 As soon as it shows any signs of this, clear it out 

 with a fine wire. Only a very small quantity is 

 required of the various chemicals — a quarter or less of the 

 quantities in the first experiment. The films should be 

 washed, as the vesicles were, and then gently crushed 

 and mounted, to be viewed with a quarter and an 

 eighth objectives and dark ground illumination. This 

 is easily managed if the objectives are either old ones 

 of small angular aperture or supplied with a movable stop 

 to reduce their larger apertures when required. The dark 

 ground illumination can be obtained with a large circular 

 stop of an achromatic condenser, or with Mr. Wenham's 

 ingenious illuminator, which gives beautiful results in the 

 hands of expert manipulators. 



Some of these film slides in the writer's collection look 

 like portions of the Milky Way as seen through a telescope, 

 or like resolvable nebulae To get these effects, the minute 

 beads in which the silica is deposited must not be piled one 

 above another, but distributed in single layers. A little 

 moving about with a needle-point is sure to get some in 

 this condition. 



Many plants and animals require silica in some part of 

 their structure, and water readily dissolves a minute 

 quantity as it acts upon soils containing it. Some sponges 

 require it to make the spicules of various shapes, by which 

 they are strengthened, or made inconvenient for their 

 assailants to bite. Other creatures deposit it in their tissues 

 like miniature cart-wheels (c/nrodota), and others like 

 anchors {s;/napta). The diatoms want it to support and 

 decorate their fine, soft tissues ; the canes and grain plants 

 to strengthen their cuticle ; and the Pottery-tree of Para 

 to make its stony bark, which the natives use in their 

 fictile manufactures. All can be accommodated, because 

 silica is not only to a certain extent soluble in water, 

 but can exist in the colloid as well as in the crystalline 

 state. All colloid or glue-like bodies differ from crj'stal- 

 loids in an important physical property. The crystalloids 

 in solution will pass freely through parchment paper, or 

 certain membranes, while the colloids remain behind. A 

 solution of water-glass — a silicate of potash, or soda — may 

 be decomposed, and a pure silica solution obtained, by care- 

 fully adding hydrochloric acid to it in a parchment paper 

 dialysing drum, and floating the drum in plain water. The 

 hydrochlorate of potash, or soda, gets through the paper, 

 and the silica stays behind.* A strong solution of water- 

 glass, limpid, like water, may be instantly turned into thick 

 jelly by adding a strong acid. 



Silica in the crystalline state forms the beautiful quartz 

 minerals, and if, when deposited from water, it had the 

 strong disposition to crystallise which many other sub- 

 stances have, it would not accommodate itself to the various 

 forms the animals and plants require. It might be worth 

 while to try various other substances besides glycerine 

 to mingle with water in film-making experiments. Some 

 interesting results would probably be obtained. It may 

 be well to add that the dealers in chemical apparatus 



* If raoro than two or threo per cent, of silica rcmaius in the 

 drum, it is likely to gelatinise. 



