W. Crookes on the Wax-paper Photographic Process. 173 



minutes, will infallibly destroy it if be 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 vapor. 



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. Varying the 

 time of exposure, within certain limits, makes very little differ- 

 ence 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 con- 

 templating,) 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 Philo- 

 sophical Magazine for March, 1855, where I recommend the 

 employment of a strong alcoholic solution of gallic acid, to be 

 dil luted 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. 



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 together ten 

 ounces of the water that has been previously used for washing 

 the excited papers (44), and four drachms of the exhausted exci- 

 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. 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 



