HOW SENSITIVENESS IS GOVERNED.
The first question likely to be asked the novice is what governs the sensitive ness of the emulsion, and this may be broadly 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 the principle is the same.) In order that this boiling may bring about the required condition of the silver it is necessary that potassium bromide (KBr) should he in excess, and an examination of the formuht should be made in order to ascertain whether that condition has been carried The equation given by chemists as 'taking place in the formation of silver bromide from potassium bromide and .silver nitrate is stated thus :- excess, it may be boiled for twenty min utes ; or with 160 grains in excess, for seven minutes. If these times are ex ceeded, the emulsion commences to show signs of fogging, any trace of which would, of course, be exceedingly injurious. The increase of sensitiveness is said to he due to the enlargement of the particles, which may be explained by the theory of crystal lisation. Just as a crystal of alum will grow in a saturated solution of the same .substance, so the potassium bromide as a solvent of the haloids will cause growth' of the particle. The particles, being
Ag.Nal KBr = AgBr KNO., 170 A 119 188 8 + ) l01 From this it will be seen that 117 parts of silver nitrate react with 119 parts of potassium bromide. fore, the 175 parts given in the first named formula will react with 122-.1 parts of potassium bromide, thus leaving 17i- grains in excess of that actually required. Now it has been found that the length of boiling is dependent upon the amount of potassium bromide present ; that is to say, the less potassium bromide in excess, the ]onger the emulsion may be boiled. For example, with only 1 grain in excess, the emulsion may he ,boiled for six hours ; with 20 grains in larger, are able to absorb more light ; and, consequently, more work may be done, and a greater amount of silver reduced, in the same time, or by the expenditure of the same energy ; therefore, the plate becomes more rapid. As this is a point that will be dealt with in a later section of the book, nothing further need be said on the subject, except that attention may be called to the fact that a. similar ex planation may be given as. to why the proportion of emulsion on the plate affects the sensitiveness.