20 



SIX LECTURES ON LIGHT. 



The aspect of these curves so fascinated 

 Faraday that the greater portion of his intel- 

 lectual life was devoted to pondering ove 

 them. He invested the space through which 

 they run with a kind of -materiality ; and th 

 probability is, that the progress of science b) 

 connecting the phenomena of magnetism 

 with the luminiferous ether, will prove these 

 " lines of force," as Faraday loved to cal 

 the magnetic curves, to represent a condition 

 of this mysterious substratum of all radianl 

 action. 



But it is not with the magnetic curves, as 

 such, that I now wish to occupy your atten- 

 tion ; it is their relationship to theoretic con- 

 ceptions that we have now to consider. By 

 the action of the bar magnet upon the needle 

 we obtain the notion of a polar forcf ; by the 

 breaking of the strip of magnetized steel, we 

 attain die notion that polarity can attach 

 itself to the ultimate particles of matter. The 

 experiment with the iron filings introduces a 

 new idea into the mind ; the idea, namely, of 

 structural arrangement. Every pair of filings 

 possesses four poles, two of which arc attrac- 

 tive and two repulsive. The attractive poles 

 approach, the repulsive poles retreat ; the 

 consequence being a certain definite arrange- 

 ment of the particles with reference to each 

 other. 



Now, this idea of structure, as produced 

 by polar force, opens a way for the intellect 

 into an entirely new region, and the reason you 

 are asked to accompany me into this region 

 is, that our next inquiry relates to the action 

 of crystals upon light. Before I speak of 

 this action, I wish y^u to realize the process 

 of crystalline architecture. Look then into a 

 granite quarry, and spend a few minutes in 

 examining the rock. It is not of perfectly 

 uniform texture. It is rather an agglomera- 

 tion of pieces, which, on examination, pre- 

 sent curiously-defined forms. You have there 

 what mineralogists call quartz, you have 

 felspar, you have mica. In a mineralogical 

 cabinet, where these substances are preserve.! 

 separately, you will obtain some notion of 

 their forms. You will see there, also, speci- 

 mens of beryl, topaz, emerald, tourmaline, 

 heavy spar, fluor-spar, Iceland spar possibly 

 a full-formed diamond, as it quitted the hand 

 of .Nature, not yet having got into the hands 

 of the lapidary. These crystals, you will ob- 

 serve, are put together according to law ; 

 they are not chance productions ; and, if 

 you care to examine them moro minutely, 

 you will find their architecture capable of 

 being to some extent revealed. They split 

 in certain directions before a knife-edge, ex- 

 P" sing smooth and shining surfaces, which 

 are called planes of cleavage ; and by follow- 

 ing these planes you sometimes reach an in 

 ternal form, disguised beneath the external 

 form of the crystal. Ponder these beautiful 

 edifices of a hidden builder. You cannot 

 help asking yourself how they were built ; 

 and familiar as you now are with the notion 



of a polar force, and the ability of that force 

 to produce structural arrangement, your in- 

 evitable answer will be, that those crystals 

 are built by the play of polar forces with 

 which their ultimate molecules are endowed. 

 In virtue of these forces, atom lays itself to 

 atom in a perfectly definite way, the final 

 visible form of the crystal depending upon 

 this play of its molecules. 



Everywhere in Nature we observe this 

 tendency to run into definite forms, and 

 nothing is easier than to give scope to this 

 tendency toy artificial arrangements. Dis- 

 solve nitre in water, and allow the water 

 slowly to evaporate; the nitre remains, and 

 the solution soon becomes so concentrated 

 that the liquid form can no longer be pre- 

 served. The nitre-molecules approach each 

 other, and come at length within the range 

 of their polar forces. They arrange them- 

 selves in obedience to these forces, a minute 

 crystal of nitre being at first produced. On 

 this crystal the molecules continue to deposit 

 themselves from the surrounding liquid. The 

 crystal grows, and finally we have large 

 prisms of nitre, each of a perfectly definite 

 hape. Alum crystallizes with the utmost 

 ease in this fash. on. The resultant crystal 

 s, however, different in shape from that Ot 

 nitre, because the poles of the molecules are 

 differently disposed; and, if they be only 

 nursed with proper care, crystals of these 

 substances may be caused to grow to a great 

 ize. 



The condition of perfect crystallization is, 

 that thi crystallizing force shall act with de- 

 ibera^ion. There should be no hurry in its 

 operation; but every molecule ought to be 

 Dermitted, without disturbance from its neigh- 

 >ors, to exercise its own molecular lights. If 

 he crystallization be too sudden, the regu- 

 arity disappears. Water may be saturated 

 vith sulphate of soda, dissolved when the 

 vater is hot, and afterward permitted to cool. 

 When cold, the solution, is supersaturated ; 

 hat is to say, more solid matter is contained 

 n it than corresponds to its temperature. 

 Still the molecules show no signs of building 

 hemselves together. This is a very remaik- 

 able, though a very common fact. The 

 molecules in the centre of the liquid are so 

 lampered by the action of their neighbors 

 hat freedom to fohow their own tendencies 

 s denied to them. Fix your mind's eye upon 

 a molecule within the mass. It wishes to 

 unite with its neighbor to the right but it 

 vishes equally to unite with its neighbor to 

 he left ; the one tendency neutralizes the 

 >ther, ana it unites with neither. We have 

 nere, in fact, translated into molecular action 

 he well-known suspension of animal volition 

 jroduced by two equally inviting bundles of 

 lay. But, if a crystal of sulphate of soda be 

 Iropped into the solution, the molecular in- 

 lecision ceases On the crystal the adjacent 

 molecules will immediately precipitate them- 

 selves; on these again others will be precipi- 



