M. TECHNOLOGY. 477 



emulsification have since been most persistently investigat- 

 ed. The first difficulty in the process lay in the inability to 

 introduce the silver nitrate and the bromide employed into 

 the collodion in quantities exactly chemically equivalent ; 

 and if the silver nitrate happens to be in excess, fogging the 

 negative results, unless some restraining mineral acid is in- 

 troduced ; while excess of the bromide reduces the sensitive- 

 ness of the film to a degree corresponding to that excess, 

 and also exerts a solvent action on the invisible image, un- 

 less it is quickly developed. In practice it was found best, 

 therefore, to employ a decided excess of bromide, and then 

 to impart sensitiveness to the film, and prevent any solvent 

 action on the image by washing out the soluble bromide 

 with water. The soluble nitrate of the base, previously 

 combined in the bromide, formed in the film in~the operation, 

 although of little consequence in the silver bath in the old 

 process, would also tend to split the film on drying by its 

 crystallization, if allowed to remain in it. All these difficul- 

 ties were, finally, simply and eftectually met by Mr. Bolton 

 by washing out all the crystallizable salts from the emulsion, 

 previously to the formation of a film Avith it on glass, while 

 a preservative agent was added at the same time. An emul- 

 sion was prepared, with only such a degree of care as to the 

 adjustment of the salts that it might give tolerable results, 

 and was then poured into a large flat dish, and the film, when 

 set, was washed several times with small quantities of dis- 

 tilled water, after having been divided into small squares 

 with a silver fruit-knife, until all soluble matter was re- 

 moved. The very light pellicle, formed by drying the film 

 obtained in this way, contains all the essential ingredients 

 fiDr a sensitive emulsion, seems to keep indefinitely, and only 

 requires simple solution in a mixture of equal parts of alco- 

 hol and ether, with the addition of a little alcoholic solution 

 of tannin and of a saturated alcoholic solution of soap, to fur- 

 nish an emulsion for the preparation of plates for present or 

 future use. Such washed emulsion pellicle, in the form of 

 shreds or powder, can be obtained by mail. The develop- 

 ment of a plate is efiected by wetting the surface with 

 methylated alcohol and rinsing with water, then treating it 

 with a weak solution of pyrogallic acid for about a minute, 

 and then, whether a feeble picture be visible or not, pouring 



