258 Mr. Talbot on a new Pi'opei'ty of the Iodide of Silvei\ 



fers from the substance of the electrical organ only by its greater 

 quantity of water. 



It would be impossible for me not to point out the analogy 

 which exists between the composition of the cerebral matter 

 and that of the electrical organ of the torpedo which we have 

 analysed.* 



XL. On a new Properti/ of the Iodide of Silver, By H. F. 

 Talbot, Esq., F.R,S.\ 

 TT is well known that certain metallic oxides and salts have 

 -■- the property of changing their colour when heated, and 

 recovering it again when cold. 



The iodide of silver affords an extremely remarkable in- 

 stance of this, and yet I believe the fact is not mentioned by 

 any chemical author. I have no doubt, therefore, that a short 

 notice of it may possess some interest. 



Let a sheet of white paper be washed over with a solution 

 of nitrate of silver, and afterwards with a rather dilute solu- 

 tion of hydriodate of potash. It will immediately assume a 

 pale yellow tint, owing to the formation of iodide of silver. 

 The paper may then be dried and laid aside for use. 



When the property in question is to be exhibited the paper 

 is held for some moments before a hot fire, and its colour 

 changes from a pale primrose tint to a rich gaudy yellow 

 emulating the sunflower. 



Removed from the fire this bright colour gradually fades 

 away, and in three or four seconds it is entirely gone. It 

 may then be reproduced, and again destroyed as before, and 

 so on for any number of times, for the heat causes no altera- 

 tion in the substance experimented upon. 



When the paper is warm and very yellow, if the finger is 

 pressed upon it and quickly removed, it leaves a print or 

 impression of its shape, v/hich is nearly white. The cause of 

 this is, that the finger is a much better conductor of caloric 

 than the atmospheric air, and therefore cools the paper in an 

 instant of time. Any cold substance may be substituted for 

 the finger ; and the effect can be produced at a little distance, 

 without actually touching the paper, merely by the radiation 



it is void of taste, of a faint {fade) smell, and saponifies with potash; if 

 burnt and calcined in a platina crucible, it leares an acid cinder, and treated 

 with boiling nitric acid it yields traces of the sulphuric and phosphoric 

 acids. It is therefore cerebral stearine. 



[• A partial chemical examination of the electrical organs of the Torpedo 

 was made by Dr. J. Davy, and recorded in his paper in Phil. Trans, for 

 1832, p. 267, an abstract of which was given in Lond. and Edinb. Phil. 

 Mag., vol. i. p. 67- — Edit.] 

 t Communicated by the Author 



