April 17, ISSa.] 



♦ KNOWLEDGE ♦ 



319 



The reader who ^\^ll make a few experiments may de- 

 termine for himself how far the motions are of the leaping 

 sort. Gamboge, for example, as I see it, has its particles 

 in a constant fidget, never for a fraction of a second ijuiet, 

 bvit varying its motions in extent. All the particles look 

 round, and it makes little or no ditiVrence whether they 

 are some distance apart or close together. The term 

 "leaping" suggests intermittence — a pause and a jump, but 

 it may certainly apply to a sudden extension of range. 

 There are no pauses. Professor Jevons' paper says, " The 

 distance through which a particle moves at one bound is 

 usually less than 1-5, 000th ])nrt of an inch, and those of a 

 grater diameter than l-5,000th of an inch are seldom seen 

 to move." Sudden extensions of the spaces traversed by 

 any particle are common enough, and they appear like 

 what anyone may imitate on a large scale by making a 

 little dance on a square foot of carpet, and every now and 

 then moving a yard. 



Very fine pumice-stone dust presents a diflPerent aspect 

 from gamboge. Many of its mobile particles are elongated, 

 and quiver as they go. 



If a penknife is rubbed on a fine hone, moistened with 

 water, and a very little of the product of the friction 

 transferred to a clean water-drop on a glass slide, it will 

 be found, on magnification, that the hone has supplied 

 coarse, angular, rocky particles, and the steel very much 

 smaller ones, which exhibit the movements. A drop of 

 old common ink was one of Professor Jevons' favourite 

 objects. He foiind its particles " in such rapid motion as 

 to cause a boiling or swarming appearance." This kind of 

 motion is frequently seen when living organisms discharge 

 a multitude of germinal, or other particles. Fine, pure 

 China clay Jevons found the most convenient for experi- 

 menting with. 



Light and darkness made no difference in the results he 

 obtained, and he was unable to confirm the opinion that 

 heat accelerated the motions. " A mixture of charcoal- 

 powder and boiled water he surrounded with ice, and a 

 similar mixture was placed in boiling water, and maintained 

 at 100 C. At the end of an hour the heated mixture had 

 deposited nearly all the charcoal, whereas the ice-cold water 

 had as much in suspension after eight hours." China-clay 

 gave the same results. 



If the particles are made small enough in proportion to 

 their specific gravity, to neutralise their tendency to sink, 

 their motion may go on for days, months, and even years, 

 if evaporation is prevented. Substances containing silica 

 are readily susceptible of the motion. Oxides, chalk, fluor- 

 spar, galena, and many others have less tendency to it. 

 Mineral acids, salts, and most soluble substances arrest 

 the motion of china clay, and are generally unfavourable 

 to the action ; but gum arable in weak solution augments 

 it Pure distilled, or rain, water is much more favour- 

 able to it than water holding minerals or chemicals in 

 solution. 



Professor Jevons traced a curious connection between the 

 state of the water most favourable to pedesis and that which 

 produced the best results with Armstrong's electric boiler. 

 In the best form of that instrument — of which a fine 

 specimen was at one time shown at the Polytechnic — 

 electricity is generated by the friction of water-particl<^s 

 escaping with high-pressure steam through nozzles of 

 partridge-wood. Details will be found in Noad's " Manual 

 of Electricity," Part I. " Faraday," he says, " found that 

 electricity is never excited by the passage of pure steam, 

 and is only manifested when water is at the same time 

 present" Pure water, as in the case of pedesis, answered 

 best, and " the smallest drop of sulphuric acid, or a little 

 crystal of sulphate of soda, dissolved in water, prevented 



the evolution of electricity." So remarked Jevons, citing 

 Faraday. 



If many experiments are contemplated, a very small 

 agate pestle and mortar should lie provided. This is the 

 best for tine grinding, but when very hard substances are 

 operated upon, a little of the aguto is ground up with them. 

 Of the substances I liavo tried, fine particles of antimony 

 have jiroved the most lively, and though very minute ones 

 are most active, those large enough to sink rather quickly 

 still show the peculiar motion. 



If we pass from small experiments under the microscope 

 to a consideration of large o])er:itions in nature, we see that 

 pedesis produces very important results. A huge river, for 

 example, like the JIis8issii)pi, brings down in the flood 

 season millions of tons of mud a day, and pedesis 

 materially extends the time it takes for the smaller 

 particles to fall and constitute a submarine dejiosit. 

 If the water of the mighty river were unfavourable 

 to their influence, the deposits would occur more 

 rapidly, and the finest strata would be much coarser than 

 under the existing conditions. The longer the fine particles 

 are suspended, the further they are carried by the .stream, 

 and all the coarser have fallen before they reach their final 

 destination. Under a great variety of circumstances the 

 pedetic movements must facilitate chemical and other 

 changes. They are doubtless subservient to digestion, 

 assimilation, and growth, and also to dissolution and decay. 

 They assist in the life processes of the smallest organism, 

 and they regulate the deposition of vast geological forma- 

 tions, such as fine slates, shules, and lithographic stones. 



OPTICAL RECREATIONS. 

 By a Fellow of the Eoyal Astronojiic.vl Society. 



{Continued from p. 1G7.) 



BEFORE proceeding to deal with the phenomena of 

 colour proper, we may conclude this portion of our' 

 subject by some mention of those of Diffraction and Inter- 

 ference, especially as they may be well illustrated by curious 

 and beautiful experiments, made with simple materials 

 within the reach of everybody. Moreover, such exi)eri- 

 ments possess a high theoretical value ; inasmuch as they 

 supply crucial tests of the truth or falsehood of the two 

 hypotheses which have been framed to explain the nature 

 of light itself ; and to which reference will be found on 

 p. 352 of our fifth volume. As there stated, Newton held 

 that light consisted of actual material elastic particles 

 emitted by the luminous body ; and alleged that, if the 

 now universally accepted theory that light has its origin in 

 the undulations of something ("the a;ther ") which fills all 

 space were true, shadows could not exist ; for that the 

 ligh^ waves would bend round the edges of opaque bodies 

 and set the aether in motion behind them ; a thing which — 

 as he asserted — never happens. He was riglit so far in his 

 theory, but wrong as to his facts ; for this bending of light, 

 partly, as it were, round a corner, actually does take place, 

 and such bending is known as Diffraction. It is with 

 some of the phenomena to which it gives rise that we have 

 now to deal. 



For their study we require either a physical point of 

 light (like a star, or the image of the sun formed by a 

 lens of short focus), or an extremely thin line of light, 

 such as is obtained through a very narrow slit brightly lit 

 by the .sun ; or, better still, from the image of such a slit in 

 a strong cylindrical lens. We will first employ a point of 

 light, and commence with our most elaborate form of 



