M. C. Lea— Sensitiveness to Light of Salts of Silver. 369 



as a columbate of the cerium oxides arid lime; whether or 

 not a neutral columbate. further examination is to determine. 



Hatcheilolite is doubtless a neutral columbate of uranium oxide 

 and lime. Then, when we pass to the samarskite, we have a basic 

 columbate of iron, uranium and yttrium oxides, Yt'rotnnta- 

 lite and euxenite are basic columbates, the former anhydrous 

 when pure, the latter hydrated, both being basic columbates of 

 Yttria and uranium oxides. 



Fergusonite is a hydrous basic columbate of yttria : and 

 Rogersite is a hydrated columbate of yttria, still more basic. 



In making this general review of these minerals, of course I 

 have not regarded those variable constituents existing in small 

 quantities and replacing more or less one or other of the pre- 

 ponderating bases of each species. 



The analysis and <■!>> miod st«dy of the above Minerals. 



I have several points under this head to consider and to 

 describe in detail ; but it more properly belongs to another 

 paper, which will supplement this, for I am now studying the 

 earthy oxides associated with the yttria of the North Carolina 

 minerals, which I am far from being satisfied contains cerium 

 oxide, although some of the reactions of cerium oxide are 

 obtained, a fact already mentioned in a note to the description 

 of samarskite. I am separating the earths from one kil - 

 the mineral, which when obtained will furnish material enough 

 for the proper solution of this question. 



Salts of silver may exhibit sensitiveness to light in three 

 several ways. They may exhib- -. or they 



may receive a latent image, and this latent image may have a 

 capacity of being rendered visible either by receiving a deposit 

 lie silver or by decomposition by alkalies in connection 

 with reducing agents! These two last mentioned phenomena 

 are verv distinct°in their character, for in the first of them the 

 ima-e is produced entirely bv the addition of silver i 

 ously present. In the second, no a added, but 



that portion of substance which received the direct action ot 

 light, undergoes dec l uen * treatment. In 



portions acl " ■ more a , pt 



to attract a precipitate in the act ot formation (tor example, 

 --■■ ^,'.-. ■ -. •'■ . . * ' '': ~ ■•" 



are more readily attacked by certain reducing agents. 



