﻿Intelligence and Miscellaneous Articles, 549 



formably on the east upon a fine-grained white oolitic marble of con- 

 siderable thickness. At the divisional line between the two forma- 

 tions many Ammonites were said to occur ; and three of these were 

 doubtfully identified with A. giganteus, hiplex, and annulatas. These 

 species would indicate the deposit to be probably of Portlandian 

 age. 



The plain of Antequera was considered by the author to consist 

 of Tertiary formations. One of these, at the south of the city, he 

 regarded as analogous to the "Calcaire grossier." He mentioned 

 indications of the presence in the vicinity of a Miliolitic marble, and 

 of a limestone containing Nummulites. Between Antequera and 

 the Torcal, he noticed a small calcareous deposit containing many 

 forms of Gryplnxa. The paper was illustrated by photographs of 

 two scenes on the Torcal, and of several species of Ammonites. 



LXX. Intelligence and Miscellaneous Articles, 



ON THE SENSITIVENESS TO LIGHT OF THE SILVER HALOID COM- 

 POUNDS, AND THE CONNEXION OF THE OPTICAL AND THE CHE- 

 MICAL ABSORPTION OF LIGHT. BY C. SCHULTZ-SELLACK. 



•TUIE haloid compounds of silver, which share with so many other 

 -*- substances the property of being altered by light, are remarkable 

 from the fact that they are photographically excited by light; that is 

 to say, they have the property of condensing mercurial vapour from 

 the air (Daguerreotype), or silver when being deposited from a liquid 

 (photography). The photographic excitation is due to the chemical 

 change ; but as it occurs before the products of decomposition of the 

 silver-salts can be shown by other means, the photographic process 

 is especially fitted for investigating the chemical action of the various 

 spectral colours on the haloid compounds of silver*. All luminous 

 rays which act chemically upon a substance are absorbed, as was 

 shown by Draper ; since chemical and optical rays of the same re- 

 frangibility cannot be separated, or rather are identical, they, of 

 course, disappear as regards the eye. Hence by the photographic 

 process we obtain absorption of light joined with chemical action, 

 chemical absorption of light. Chloride, bromide, and iodide of silver, 

 as I have previously shownf, may by melting be obtained in perfectly 

 clear masses ; in this condition they are only very slowly changed by 

 light, and are especially suited for investigating optical absorption. 



I have found that optical and chemical absorption of light exactly 

 coincide. All colours which in a thickness of some millimetres exert 

 an appreciably optical absorption, effect chemical decomposition ; ab- 

 sorption of light in these substances is always connected with chemical 

 action. 



Iodide, bromide, and chloride of silver are sensitive to the ultra- 



* In what follows, the usual collodion process was used : collodion mixed 

 with dissolved haloid salts was poured upon a glass plate, the plate dipped 

 in solution of silver, and after exposure developed by means of ferrous 

 sulphate. 



f Pogg. Ann. vol. exxxix. p. 182. 

 Phil. Mag. S. 4. No. 276. Suppl. Vol. 41. 2 



