CHEMICAL EEACTIONS OF THE CYANIDE BATH.
The exact method of compounding the bath is considerably more important than might at first appear. There are eertain points to consider, which must be carried out with equal care in various baths. The first of these is that the gold must be added last, as noted in each of the for mulae Secondly, that within certain well defined limits, a slight alteration in the amount of sulphocyanide is not im portant. When gold solution is added to the solution of sulphocyanide, it first of all changes to a bright red, and after wards becomes clear again. The changes are almost instantaneous, or should be if the bath has been properly made ; but they are sufficiently marked to be ob servable by every printer. This indicates that some new compound is formed on the addition of the gold, which is afterwards redissolved, and such is found to be the case, as shown in the formula Au CI, + 3 Am Cy S = Au (Cy + 3 Am Cl (Gold trichloride and ammonium sulpho• cyanide give gold sulphocyanide and am monium chloride.) Now, gold sulpho cyanide is a bright red compound, so that this is what has been formed in the solution. It is redissolved in excess of ammonium sulphocyanide, giving a double salt of gold-ammonium sulphocyanide, having the abbreviated formula An (Cy S) ,Am Cy S. (In all these remarks it may be taken that the potassium salt may be substituted for the ammonium, wherever preferred, and used in exact proportion to its chemical equivalents.) Fuller informa
tion on the chemistry of the process is given in the theoretic section. This is the substance which it is desirable to form. It follows, then, that sufficient of the salt must be used to accomplish the purpose of redissolving the precipitate. The exact quantity required is easily estimated by the molecular or combining weights, and will be found to work out as 3 is to -1. That is to say, 3 grains of gold require 4 grains of sulphocyanide. Now, many fornmhe give a much greater quantity, and it has been found by experiment that practically no difference occurs between the use of 5 grains to 10 grains to each grain of gold. Above this, however, there is, with certain papers, a danger, but this depends on (rc) the temperature, and (5) the melting point of the gelatine used for the paper. When the solution contains too much sulphocyanide. it attacks the gelatine, softening it, and making it peculiarly liable to abrasion and finger marks. With some brands of paper, 15 to 120 times the amount of gold may be used, even in warm weather ; but with others it is not advisable to go above 10 times. The necessary modifications of the formula may be easily determined from the infor mation already given.