. The book of photography; practical, theoretical and applied. ne No. 1 soluble, 30 grs., water 1 oz. ;(6) silver nitrate IVS grs., water i oz. ; (c) potassium bromide 130 grs., water 1 oz. ; (d) potassium iodide 5 grs., water 1 oz. ;() hard gelatine 240 grs., water 2 oz. How Sensitiveness is Governed. The first question likely to be asked bythe novice is what governs the sensitive-ness of the emulsion, and this may bebroadly stated to be the ripening or boil-ing to which it is subjected. (There are,however, other methods of emulsion mak-ing which do not necessitate boiling;which will be dealt
. The book of photography; practical, theoretical and applied. ne No. 1 soluble, 30 grs., water 1 oz. ;(6) silver nitrate IVS grs., water i oz. ; (c) potassium bromide 130 grs., water 1 oz. ; (d) potassium iodide 5 grs., water 1 oz. ;() hard gelatine 240 grs., water 2 oz. How Sensitiveness is Governed. The first question likely to be asked bythe novice is what governs the sensitive-ness of the emulsion, and this may bebroadly stated to be the ripening or boil-ing to which it is subjected. (There are,however, other methods of emulsion mak-ing which do not necessitate boiling;which will be dealt with later, but theprinciple is the same.) In order that this PLATES AND FILMS. 61 boiling may bring about the requiredcondition of the silver it is necessary thatpotassium bromide (KBr) should be inexcess, and an examination of the formulashould be made in order to ascertainwhether that condition has been carriedout. The equation given by chemists astaking place in the formation of silverbromide from potassium bromide andsilver nitrate is stated thus : —. excess, it may be boiled for twenty min-utes ; or with 160 grains in excess, forseven minutes. If these times are ex-ceeded, the emulsion comm-snces to showsigns of fogging, any trace of which would,of course, be exceedingly injurious. Theincrease of sensitiveness is said to be dueto the enlargement of the particles, whichmay be explained by the theory of crystal-lisation. Just as a crystal of alum willgrow in a saturated solution of the samesubstance, so the potassium bromide asa solvent of the haloids will cause growthof the particle. The particles, being
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