OPTICS. 



let, and finally a beautiful blue of the 

 second order. Hence, as M. Biot re- 

 marks, we observe that the extinction to 

 which the intensity is accidentally subject, 

 does not prevent the tints from follow- 

 ing the same order as that of the rings. 



A great number of the metallic oxides 

 exhibit a momentary change of tint by 

 being heated, and resume their primi- 

 tive tint by cooling. This arises from 

 the increase of size in the particles, 

 and consequently the new colours thus 

 developed should rise in the order of 

 colours.* A phenomenon of the oppo- 

 site kind was observed by M. Chevreul 

 in volatilizing indigo spread upon paper. 

 During vaporization the indigo colour 

 passes into a poppy red, highly brilliant, 

 which seems to prove that the particles 

 have become less in the act of evapora- 

 tion. The same eminent chemist no- 

 ticed an analogous fact in the new sub- 

 stance, which he calls hcematine. This 

 substance, when pure and solid, has a 

 greyish tint. When dissolved in water, 

 containing some drops of acetic acid, it 

 produces a fluid, whose colour is a 

 slightly greenish yellow of the second 

 order. If the fluid in this state is in- 

 troduced into a tube filled with mercury, 

 and heated by surrounding it with a hot 

 iron, it becomes successively yellow, 

 brilliant orange, brilliant red, purple, 

 and bluish purple; and, what is very 

 remarkable, if it is afterwards left to 

 cool, it returns gradually to its primitive 

 tint, which it requires some days to do, 

 if the quantity used is about the one- 

 third of a cubic inch. 



The progressive steps by which bo- 

 dies attain their definite tints, are well 

 seen in the crystallization of a saturated 

 solution of super-oxygenated muriate of 

 potash during its slow cooling. As the 

 temperature falls, the salt is precipitated 

 in thin rectangular scales which unite 

 to one another, and whose thinness is 

 such, that they are differently coloured, 

 according to the obliquity of the inci- 

 dent light, or the thickness of the scales. 

 The thickest are of an uniform white 

 colour, and the thinnest, by uniting 

 themselves to others, become white in 

 their turn. Sometimes they do not ap- 

 ply themselves exactly to one another, 

 and then the}' do not cease to reflect Ihe 

 tints which they exhibit individually, 

 even though they form part of a plate 

 too thick to produce these colours. 

 Similar variations are seen in the small 



* The tints described by M. Gay Lussac in the 

 ^.nn. de Chimie, follow the order of the rings. 



scales of acidulous tartrite of potash 

 precipitated from a warm and saturated 

 solution of this salt.* 



5. The vegetable kingdom presenls 

 many curious illustrations of Newton's 

 theory, as he himself observed, and as 

 we have noticed in Prop. 7. M. Biot is 

 of opinion, that the colours descend-!' 

 in the order of rings, as the force of 

 vegetation developes itself, and ascend 

 during its decay. The young buds of 

 the oak and of the poplar, for example, 

 are at first of a red colour, bordering 

 on orange ; from this they pass to a 

 reddish orange, and soon to a green, 

 through a kind of reddish yellow, ex- 

 tremely fugitive. When the flower of 

 the honeysuckle blows, its colour is a 

 pure white of the first order, and in de- 

 caying it passes into pale yellow, yellow, 

 orange, and deep orange. The flower of 

 the geranium sanguineum, whose colour 

 is a violet red, intermediate between the 

 first and second order, becomes blue in 

 withering. Pinks of a bright red of 

 the second order pass as they decay into 

 a poppy red, and a violet purple. The 

 same thing happens to certain species of 

 roses, but ^here are others whose colour 

 appears to be red of the third order. 

 While these grow old upon their stalk, 

 they lose by degrees the brilliancy of 

 their red, and the blue and violet of the 

 fourth order, acquiring a greater in- 

 fluence over their tints, they rise to a 

 bluish red. The tigridia, which blows 

 and withers in a tew hours, appears, 

 even when it is not quite open, of a 

 bright reddish orange, from which it 

 rises to a deep red oft he first order, and 

 in withering it rises to the violet red of 

 the second order. The cobcca when it 

 opens is at first of a pale and imperfect 

 yellowish green, of the second order ; 

 but it is soon spotted with violet, and in 

 a few hours it becomes wholly violet, 

 without passing through the interme- 

 diate blue. In withering, however, it 

 descends from violet to blue. M. De- 

 candolle ascribes the sudden change of 

 colour at the first period to the fecun- 

 dation, which he considers as the cause 

 which modifies rapidly the colour of a 

 great many flowers. % 



* Sen Blot's Trait.c dc Physique, torn. iv. p. 133. 



t In quoting the opinions of this eminent philoso- 

 pher, it is necessary to state, th;it when he uses the 

 word ascend in the order of rings, we use descend, 

 because the colours fall from a higher to a lower 

 order. M. Biot's term ascend, indicates a local as- 

 cent in the printed table, the first order being at the 

 top of the table, and the last order at the bottom. 



; Biot's Traite de Physique, torn. iv. p. 133. 



