PREPARATION OF THE. PAPER.
The first consideration is the choice of a suitable paper, and for experimental work there is nothing bettdr than What man's, hot-pressed drawing, paper. Any pure paper may be employed, avoiding those with glazed surfaces and those which appear to be sized with impute gelatine. Excellent results have been produced on ordinary typewriter paper. The next point to consider is as to whether the chloride or the silver solution shall be employed first, When a solution, say sodium chloride, is added to silver nitrate, silver chloride and sodium nitrate are formed, as expressed by the equation Nael AgC1 NaN 03.
Atomic weights : 108 + 1 + (16 X 3) + + 35 5 Molecular : 170 58'5 2 2or asis to 340 117This is the action which takes place in the making of the paper. Chemists have found that certain substances react always in the same proportions to form com pounds, without relation to the actual weights of the substances that may be in contact. To make this clearer, take the atomic weights in the equation just given. These are shown by the top row of figures, and the totals give the molecular or com bining weights. Thus, it will be seen that parts of the one always react with 117 parts of the other, so that if equal quantities by weight of each were used, one substance would be in excess of the other. When silver
chloride is exposed to light, chlorine is liberated ; this can be proved by taking a sample of the substance and noting the smell of chlorine gas given off on exposure to light. When some substance is pre sent which is an absorbent of chlorine, the decomposition takes place more readily, for two agencies are at work—the light tending to liberate the chlorine and the absorbent ready to combine with the chlorine. Now supposing the substanc. which is in excess to be chlorine, the action can only proceed slowly, for chlorine is not an absorbent of itself. If the substance in excess be silver nitrate, the chlorine is readily taken up ; therefore the silver nitrate, or sensitising solution, must be used last, so that the chlorine may all be used up, and the solution remaining on the surface of the paper which is in excess of that actually necessary for the forma tion of silver chloride should be silver nitrate.