OPTICS. 



G3 



opaque. If we now saturate it with 

 water, the pore A B C D will be com- 

 pletely filled : the two great refractions 

 which took place at F and G will no 

 longer exist, and the light will suffer 

 only a slight refraction at E and H, by 

 which it will be less scattered than 

 when the tabasheer was dry, a result 

 which is perfectly conformable to ob- 

 servation. 



An analogous effect is produced with 

 opaque tabasheer and oil. This opaque 

 variety, which retains its opacity when 

 its pores are filled with water, acquires 

 the most beautiful transparency by the 

 absorption of oil of beech nut. Hav- 

 ing saturated a large piece of this va- 

 riety witli oil of beech nut coloured red 

 with anchusaroot, it was laid on amass 

 of lead of a lower temperature than that 

 of the room. The oil instantly appeared 

 to retire from the surface into its inte- 

 rior, and the transparent mass became 

 opaque like a piece of red brick. Upon 

 removing it into its former temperature 

 the tabasheer resumed its transparency. 

 In this experiment the oil seems to have 

 quitted the centre of the pores in con- 

 sequence of its contraction by cold, and 

 collecting itself by capillary- attraction 

 round the solid parts, left the pores in 

 the state shown in fig. 55. The appli- 

 cation of heat, by expanding the oil, 

 causes it to fill the pores and resume its 

 transparency. If when the tabasheer 

 is saturated with oil, it is carried into a 

 warmer place, a part of the oil will be 

 discharged by expansion. 



Tabasheer presents us witha still more 

 remarkable property relative to the three 

 conditions of solid bodies, viz. transpa- 

 rency, and black and white opacity. If 

 we wrap a transparent piece in paper, 

 and burn the paper, and repeat this ope- 

 ration twice or thrice, the tabasheer 

 will become perfectly black and opaque, 

 with a sort of pitchy lustre. A red 

 heat will restore it to its primitive state ; 

 but if the heat is considerably below 

 redness, some specimens acquire a 

 slight transparency, aul a dark slaty 

 bine colour, shading in some places into 

 whiteness. When slightly wetted in 

 this state it becomes chalky white ; 

 with a greater portion of water it be- 

 comes black, and with a still greater 

 portion it becomes again transparent.* 



3. Several curious phenomena of 

 colour are presented by mineral bodies. 



* Sec the Philosophical Transactions for 1319, 

 p. 23*. 



The yellow Brazil topaz loses its yel- 

 low colour entirely by heat, without suf- 

 fering any change in its other properties. 

 Some specimens thus become nearly 

 colourless, while others are left with a 

 fine pink colour, which is much prized. 

 The yellow phosphate of lead grows 

 green when heated. The balas ruby 

 in some specimens becomes green by 

 heat ; the green fades into brown as the 

 cooling advances, and the brown rises 

 to its original red colour, 



4. Tincture of turnsole, which be- 

 comes orange after being long corked 

 up in a bottle, an effect ascribed to de- 

 oxidation, becomes red in a few minutes, 

 and then violet blue, by opening the 

 bottle and shaking the fluid, the colour 

 thus passing from the first to the second 

 order. The Cameleon Mineral, which 

 is a bright green of the third order, is 

 a solid formed by heating pure and 

 solid oxide of manganese with potash. 

 When dissolved in much warm water, 

 it is rapidly disunited and separated 

 from the oxide ; but if a little water is 

 used, and if the mineral is well made, 

 the separation becomes progressive, the 

 solution changing its colour from green 

 to bluish green,"blue, purple and red- 

 dish purple, the last descending in the 

 order of the rings, as if the particles 

 became smaller. M. Biot conceives, 

 that the proportion of potash united to 

 the oxide, is successively dissolved by 

 the action of the water, till it is all car- 

 ried off, and the oxide alone left in the 

 liquor ; and hence he concludes, that the 

 brown (brun-marin) colour of the oxide 

 is a reddish orange, of the second 

 order, rendered excessively sombre by 

 the absorption of a great quantity of 

 light. Another chemical fact of much 

 interest was observed by M. Claubry. 

 He mixed oil of sweet almonds with 

 soap and sulphuric acid. The combina- 

 tion which is at first yellow, soon passes 

 to orange yellow and to deep orange, 

 and thence to red and to violet, which, 

 as M. Biot observes, is precisely the 

 order of colours as they advance from 

 the first to the second order. In the 

 passage from the orange to the red, 

 there is an instant when the absorption 

 of the incident rays -is so strong, that 

 the mixture appears almost black. The 

 same interruption is observed, if in 

 place of oil of almonds we use oil ob- 

 tained irom alcohol, treated with chlo- 

 rine. The colours then pass through 

 the following gradations ; pale yellow of 

 the first order, orange, black, red, rio- 



