SIX LECTURES ON LIGHT. 



21 



) and this act of precipitation will con- 

 tinue from the top of the flask to the bottom, 

 until the solution has, as far as possible, as- 

 sumed the solid form. The crystals here 

 formed are small, and confusedly arranged. 

 The process has been too hasty to a imit of 

 the pure and orderly action of the crystalliz- 

 ing lorce. It typities the state of a nation in 

 which natural and healthy change is resisted, 

 until society becomes, as it were, supersatu- 

 rated with the desire lor change, the change 

 being effected through confusion and revolu- 

 tion, which a wise foresight might have 

 avoided. 



Let me illustrate the action of crystallizing 

 force by two examples of it : Nitre might be 

 employed, but another well-known substance 

 enables me to make the experiment in a bet- 

 ter form. The substance is common sal- 

 ammoniac, or chloride of ammonium, dis- 

 solved in water. Cleansing perfectly a glass 

 plate, the solution of the chloride is poured 

 over the glass, to which, when the plate is set 

 on edge, a thin film of the liquid adheres. 

 Warming the glass slightly, evaporation is 

 promoted ; the plate is then placed in a solar 

 microscope, and an image of the film is thrown 

 upon a white screen. The warmth of the il- 

 luminating beam adds itself to that already 

 impa- ted to the glass plate, so that after a 

 moment or two the film can no longer exist in 

 the liquid condition. Molecule then closes 

 with molecule, and you have a most impres- 

 sive display of crystallizing energy overspread- 

 ing the whole screen. You may produce 

 something similar if you breathe upon the 

 frost ferns which overspread your window- 

 panes in winter, and then observe through a 

 lens the subsequent recongelation of the film. 



Here the crystallizing force is hampered by 

 the adhesion of the film to the glasys ; never- 

 theless, the play of power is strikingly beau- 

 tiful. Sometimes the crystals start from the 

 edge of the film and run through it from that 

 edge, for, the crystallization being once 

 started, the molecules throw themselves by 

 preference on the crystals already formed. 

 Sometimes the crystals start from definite 

 nuclei in the centre of the film ; every small 

 crystailins particle which rests in the film fur- 

 nishes a starting-point. Throughout the pro- 

 cess you notice one feature which is perfectly 

 unalterable, and that is, angular magnitude. 

 The spiculce branch from the trunk, and from 

 these branches others shoot ; but the angles 

 enclosed by the spiculre are unalterable. In 

 like manner you may find alum-crystals, 

 quartz-crystals, and all other crystals, dis- 

 torted in shape. They are thus far at the 

 mercy of the accidents of crystallization ; but 

 in one particular they assert their superiority 

 over ail such accidents angttlar magnitude 

 is always rigidly preserved. 



My second example of the action of crys- 

 tallizing force is this: ! y sending a voltaic 

 current through a liquid, you know that we 

 decompose the liquid, and if it contains a 



metal, we liberate this metal by the electro- 

 lysis. This small cell contains a solution of 

 acetate of lead, and this substance is chosen 

 because lead lends itself freely to this crys- 

 tallizing power. Into the cell dip two very 

 thin platLum wires, and these are connected 

 by other wires with a small voltaic battery. 

 On sending the voltaic current through the 

 solution, the 1 ad will be sl.wly severed from 

 the atoms with which it is now combined; it 

 will be liberated upon one of the wires, and 

 at the moment of its liberation it will obey 

 the polar forces of its atoms, and produce 

 crystalline forms of exquisite beauty. They 

 are now before you, sprouting like ferns 

 from the wire, appearing indeed like vegeta- 

 ble growths rendered so rapid as to be plain- 

 ly visible to the naked eye. On reversing the 

 current, these wonderful lead-fror ds will dis- 

 solve, while from the other wire filaments of 

 lead dart through the liquid. In a moment 1 

 or two the growth of the lead-trees recom- 

 mences, but they now cover the other wire. 

 In the process of crystallization, Nature first 

 reveals herself as a builder. Where do her 

 operations stop ? Does she continue, by the 

 play of the same forces, to form the vegeta- 

 ble, and afterwards the animal ? Whatever 

 the answer to these questions may be, trust 

 me that the notions of the coming genera- 

 lions regarding this mysterious thing, which 

 some have called "brute matter," will be 

 very different from those of the generations 

 past. 



There is hardly a more beautiful and in- 

 structive example of this play of molecular 

 force ti an that furnished by the case of water. 

 You have seen the exquisite fern-like forms 

 produced by the crystallization of a film of 

 water on a cold window pane. You have 

 also probably noticed the beautiful rosettes 

 tied together by the crystallizing force during 

 the descent of a snow-shower on a very calm 

 day. The slopes and summits of the Alps 

 are loaded in winter with these blossoms of 

 the frost. They vary infinitely in detail of 

 beauty, but the same angular magnitude is 

 preserved throughout. An inflexible power 

 binds spears and spiculce to the angle of 60 

 degrees. The common ice of our lakes ii 

 also ruled ki its deposition by the same angle. 

 You may sometimes see in freezing water 

 small crystals of stellar shapes, each star con- 

 sisting of six rays, with this angle of 60 be- 

 tween every two of them. 'i his structure 

 may be revealed in ordinary ice. In a su<fc- 

 j beam, or, failing that, in our electric beam, 

 I we have an instrument delicate enough to 

 I unlock the frozen molecules without disturb- 

 | ing the oider of their architecture. Cutting 

 I from clear, sound, regularly-frozen ice a sl-tb 

 i parallel to the planes of freezing, and send- 

 I ing a sunbeam through such a slab, it lique- 

 j fies internally at special points, round each 

 ! point a six-petalled liquid flower of exquisite 

 I beauty being formed. Crowds of such flow?' 

 ' ers are thus produced. 



