When two symbols are written side by side without any inter-vening sign, it is implied that the elements represented are in combination. Thus, AgCl indicates that silver and chlorine are combined together to form silver chloride (-ide being the recog-nised termination for a compound of two elements); indeed, to the chemical expert very much more is indicated, the most im-portant thing being that 108 parts by weight of silver are united with 35.5 parts by weight of chlorine.
Most chemical reactions connected with photographic practice are sufficiently understood for chemists to be able to represent them by such symbolic representation as show the quantities of materials concerned in the reactions, and it is from this point of view that the table of atomic weights is of special value. Limit of space does not allow us to give all the various rules and usages of the symbolic language by which chemists represent the quantities taking part in reactions, but an example will teach a good deal.
When a solution of potassium bromide (KBr) is mixed with a solution of nitrate of silver (AgNOs), the silver and the potas-sium change places, forming silver bromide (AgBr) - which, being insoluble, is precipitated - and potassium nitrate (KNOa), which remains in solution.
KBr + AgNOa m AgBr + KNOa.
If we wish to bring about this reaction without loss or interfer-