216 report — 1859. 



as before, taking care that each sheet be washed twice in distilled water, 

 and that at every fourth sheet the dishes of washing water be emptied, and 

 replenished with clean distilled water : this water should not be thrown 

 away, but preserved in a bottle for a subsequent operation (49). 



45. The above quantity of the exciting bath will be found quite enough 

 to excite about fifty sheets of the size here employed, or 3000 square inches 

 of paper. After the bulk has been exhausted for this purpose, it should be 

 kept, like the washing waters, for the subsequent operation of developing (49). 



Of course these sensitive sheets must be kept in perfect darkness. Gene- 

 rally sufficient attention is not paid to this point. It should be borne in 

 mind, that an amount of white light, quite harmless if the paper were only 

 exposed to its action for a few minutes, will infallibly destroy it if allowed 

 to have access to it for any length of time ; therefore, the longer the sheets 

 are required to be kept, the more carefully must the light, even from gas, 

 be excluded; they must likewise be kept away from any fumes or vapour. 



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 develop- 

 ment. Varying the time of exposure, within certain limits, makes very little 

 difference on the finished picture ; its principal effect being to shorten or 

 prolong the time of development. 



Unless the exposure to light has been extremely long (much longer than 

 can take place under the circumstances we are contemplating), nothing will 

 be visible on the sheet after its removal from the instrument, more than there 

 was previous to exposure ; the action of the light merely producing a latent 

 impression, which requires to be developed to render it visible. 



47. The developing solution in nearly every case consists of an aqueous 

 solution of gallic acid, with the addition, more or less, of a solution of nitrate 

 of silver. 



An improvement on the ordinary method of developing with gallic acid, 

 formed the subject of a communication to the Philosophical Magazine for 

 March 1855, where I recommend the employment of a strong alcoholic solu- 

 tion of gallic acid, to be diluted with water when required for use, as being 

 more economical both of time and trouble than the preparation of a great 

 quantity of an aqueous solution for each operation. 



48. The solution is thus made : put two ounces of crystallized gallic acid 

 into a dry flask with a narrow neck; over this pour six ounces of good 

 alcohol (60° over proof), and place the flask in hot water until the acid is 

 dissolved, 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 mixing togelher ten ounces of the water that 

 has been previously u<ed for washing the excited papers (44), and four 

 drachms of the exhausted exciting 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. The dish must be 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 cause a stain. Should this happen, either 

 dry it with blotting-paper, or immerse the sheet entirely in the liquid. 



50. If the paper has been exposed to a moderate light, the picture will 



