276 Mr. Hunt on Actino-Chemistiy, 



true height of the mercury in the case of a barometer. If 

 circumstances should render the correction obtained by the 

 foregoing formula of any importance, tables could be easily 

 calculated to facilitate its practical application. 



Dublin, August 14, 1845. .»;)i, . } o,i,;!i,v- 



XLII. Cojitributions to Actino-Chemistry, — The Chemical 

 Changes produced by the Solar Rays on some Photographic 

 Preparations examined. By Robert Hunt, Keeper of 

 Mining Records, Museum of (Economic Geology '^. 



1. "DEING desirous of ascertaining, with more correctness 

 -*-* than has hitherto been done, the nature of the changes 

 produced by solar radiations upon the various preparations 

 used for photographic purposes, I instituted, a short time 

 since, a series of experiments with this object in view. Com- 

 mencing with the salts of silver most commonly used in this 

 new and beautiful art, it is my intention to proceed with all 

 organic as well as inorganic bodies, which the researches of 

 late years have shown us undergo a chemical change by ex- 

 posure to sunshine. This inquiry must necessarily extend 

 itself over a considerable period, but by confining myself 

 strictly to the examination of one compound at a time, 1 hope 

 to be enabled gradually to place the entire subject in a more 

 satisfactory light than it is at present. The results I have al- 

 ready obtained are in the highest degree satisfactory; and as 

 they have reference particularly and solely to the oxides, ni- 

 trate and chloride of silver, I see no good reason for with- 

 holding their publication. As the examination of the other 

 preparations is completed, I shall, from time to time, forward 

 my results to the editors of the Philosophical Magazine. 



2. During the past five or six years, the attention of some 

 of the most eminent philosophers of Europe has been turned 

 to the subject of photography. The result has been the dis- 

 covery of a great number of most interesting processes ; and 

 in some few cases the chemistry of the changes produced by 

 ACTINIC power has been examined and explained. This has, 

 however, so rarely been the case, that I shall ofier no apology 

 for proceeding anew over the entire subject; and I hope in 

 every instance, where I do not acknowledge the previous la- 

 bours of other inquirers, that the omission will be set down to 

 its true cause — my ignorance of those labours — and not attri- 

 buted to any desire on my part to arrogate to myself the merit 

 of any discovery which is fairly due to another. 



* Communicated by the Author. 



