1814.] New Properties of Light. 191 



3. All soft substances that lose their lustre on cooling; and otliers 

 when formed into a lliin plate by heat, between two pieces of glass, 

 depolarise light in every position, 'i'he most remarkable of these 

 are white v/ax, spermaceti, adipocire from human bodies, oil of 

 mace, acetate of lead, manna, adipocire from animal muscle, adipo- 

 cire from biliary calculi, tallow, benzoic acid, oxalic acid. Of these 

 substances oil of mace exhibits a series of surprising phenomena. 

 In some parts of the plate it restores the complete image of the 

 vanished candle : in other parts it restores two condensed nebulous 

 images of the candle, separated by a small interval : and in another 

 place it depolarises or restores Jour vebiiloiis wimys, or sectors of 

 light, separated from each other by four dark and broad radial lines, 

 at the meeting of which is the place of ti>e vanished image. The 

 other phenomena presented by this remarkable substance could not 

 be rendered intelligible without the aid of a figure. 



4. The phenomena exhibited by tallow are equally interesting. 

 When first cooled, it does not display, like the other bodies, any 

 optical indications of a crystallized structure; but after standing five 

 or six days, an incipient crystallization begins to develope itself, by 

 the depolarisation of a small quantity of nebulous light. This 

 nebulosity increases, and about the 16th day four luminous sectors, 

 like those of oil oi" mace, are imperfectly developed. This effect of 

 time in completing the crystallization of bodies leads to new specu- 

 lations respecting the structure of what has been considered as mere 

 unorganized matter. 



5. Light may be polarised and depolarised in the same body, the 

 Incident pencil being common light, and the emergent pencil de- 

 polarised light. 



II. Theory 'of the Depolarisation of Light. 



When 1 first discovered this pro])erty of crystallized bodies, I 

 naturally referred it to a cause different from that of double refrac- 

 tion. The faculty of depolarising light was possessed by many 

 suljstanccs that gave no indications of double refraction, and evea 

 by animal and vegetable products, such as horn, caoutchouc. The 

 circuirntancc, however, of agate and Iceland spar having the pro- 

 perty both of polarising and depolarising light, and the constant 

 relation in the position of the axes which regulate these apjoarently 

 opposite action?^, induced me to think that the two classes of pheno- 

 mena had the same oi igln. This opinion was afterwards confirmed 

 by an experiment with a bundle of glass plates, by which the 

 vanished image v.as depolarised by polarising it in a new j)lane; but 

 in applying the principle to other phenomena, I was baffled in every 

 attempt to generalize them. By extending, however, and varying 

 the experiments, 1 have discovered the general principle to which 

 they all Ijclong, and have tlius been led to several conclusions, which 

 could not easily have been obtained by direct experiment. In a 

 »hort notice like the present, i cannot hope to muke this theory well 

 under; tood, v.ithout the details which I have given in my larger 



