226 On Gallic Acid as a Developing Agent in Photography. 



overcome : he recommends the addition of a small proportion 

 of alcohol or acetic acid to the aqueous solution, which will so 

 arrest decomposition as to render it possible to keep in stock a 

 quantity of this reagent, as we should of any other stable prepa- 

 ration. 



Having undertaken the arrangement of the department in 

 this Observatory in which photography is applied to the regis- 

 tration of the various meteorological phenomena, and having 

 for many reasons decided upon the use of the wax-paper pro- 

 cess* as that best adapted to the purpose, I have naturally been 

 anxious to avail myself of any improvements which may tend to 

 diminish the labour or contribute to the general perfection of 

 the result. 



At first the solution of gallic acid for developing the latent 

 image was prepared in large quantities at a time, and preserved 

 from decomposition by the process just referred to ; but the 

 amount of liquid necessary to be prepared at once, in order to 

 last any length of time, proved to be so bulky, that I was in- 

 duced to try whether the method could not be so far modified 

 as to admit of the water being entirely left out of the stock so- 

 lution, and only added as required for use. This led to the em- 

 ployment of alcohol as a solvent for the gallic acid ; and finding 

 that under these conditions the character of the latter as a de- 

 veloping agent was greatly heightened, I resolved to make a few 

 experiments on the subject, the result of which I beg leave 

 to communicate, in the hope that the particulars will be found 

 useful to others similarly situated to myself. 



Two ounces of gallic acid are to be dissolved in 6 ounces of 

 alcohol (60° over proof) ; to hasten solution, the flask may be 

 conveniently heated by immersion in hot water ; when cold, it 

 should be filtered, mixed with half a drachm of glacial acetic acid, 

 and preserved in a stoppered bottle for use; so prepared, it will 

 keep unaltered for a considerable length of time. The gallic 

 acid is not precipitated from this solution by the addition of 

 water ; consequently, if in any case desirable, the development cf 

 a picture may be effected with a much stronger bath than the 

 one usually employed. 



To obtain a solution of about the same strength as a saturated 

 aqueous solution, half a drachm of the above would require to 

 be added to 2 ounces of water ; but for my particular purpose 

 I prefer a weaker bath, which is prepared by mixing half a 

 drachm with 10 ounces of water. In either case it will be 

 found necessary to add solution of nitrate of silver in small 

 quantities as the developing picture seems to require it. 



Radcliffe Observatory, Oxford, 

 February 1855". 



* Notes and Queries, vol. vi. p. 443. 



