46. Experience alone can tell the proper time to expose. the 
Sensitive paper to the action of light, in order to obtain the best 
effects. However, it will be useful to remember, that it is almost 
always possible, however short the time of exposure, to obtain 
some trace of effect by prolonged development. Varyi 
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be 
Be 
ae 
An improvement on the ordinary method of developing with 
gallic acid, formed the subject of a communication to the Philo- 
sophical Magazine for March, 1855, where I recommend the 
employment of a strong alcoholic solution of gallic acid, to be 
dilluted with water when required for use, as being more econo- 
_ mical both of time and trouble than the preparation of a great 
“quantity of an aqueous solution for each operation. 
~ 8, The-solution is thus made: put two ounces of crystallized 
gallic acid into a dry flask with a narrow neck; over this poms 
_ 81x ounces of good alcohol, (60° over proof,) and place the flask 
in hot water until the acid is Sroclnea or nearly so. This will 
not take long, especially if it be well shaken once or twice. 
__ Allow it to cool, then add half a drachm of glacial acetic acid, 
_ and filter the whole into a stoppered bottle. 
_ 49, The developing solution which I employ for one set of 
sheets, or 180 square inches, is prepared by aoneg together ten 
ounces of the water that has been previously for washing 
the excited papers (44), and four drachms of the exhausted exei- 
_ ting bath (45); the mixture is then filtered into a perfectly clean 
dish, and half a drachm of the above alcoholic solution of gallic 
acid poured into it. dish > shaken about until the 
greasy appearance has quite gone from the surface; and then the 
sheets of paper may be laid down on the solution in the ordinary 
Manner with the marked side downwards, taking particular care 
that none of the solution gets on the back of the paper, or it will 
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