In order to do good photographic work by method rather than by guess, it is necessary to understand something of the nature of the chemicals used and their effects. Even a slight knowledge of chemistry enables the amateur to work understandingly and with far better results. We are therefore going to give, for the benefit of our Camera Club, a few papers on chemistry as used in photography, and shall try to make them so plain and simple that even the youngest member will understand them.
Chemistry is that science which explains the composition of the substances which compose the crust of the earth, the atmosphere which surrounds it, and the water which occupies so much of the earth's surface. These substances are called chemical elements. A chemical element is a simple substance containing only one kind of matter, such as gold, silver, platinum, hydrogen, oxygen, etc. According to the last report of Mr. F. W. Clark, the chief chemist of the U.S. Geographical Survey, there are seventy-two known elements. About thirty of these elements are used in the different processes of photography.
Each element is represented by a symbol, this symbol being the first letter or letters of the name of the element. The symbol of hydrogen is "H"; of oxygen is "O"; of gold, "Au," the first two letters of the word "Aurum," the Latin name for gold. Each symbol also stands for the weight of one of its atoms. (An atom is supposed to be the smallest possible division of a substance.) Hydrogen is the lightest element known, and is taken as the standard of weight when comparing the weight of other atoms. The symbol "H" would therefore not only stand for the element hydrogen, but for its weight, 1, or a unit. An atom of oxygen is sixteen times as heavy as an atom of hydrogen, and an atom of gold is 196 times as heavy.
In making up chemical compounds the chemical elements are combined in different proportions, which, united, make a new substance. The way in which these elements combine is always in the same proportion. The smallest number of atoms which combine to form a new substance is called a molecule. Take water, for instance, which is composed of hydrogen and oxygen; it takes two atoms of hydrogen and one atom of oxygen to form a molecule of water. These chemical combinations are expressed or written by the symbols of the elements of which they are composed, called chemical formulas. If two or more atoms of an element are used to form a chemical compound, the number of atoms used is written directly after the symbol; thus, H2O is the chemical formula for water.
Two well-known developing agents, pyrogallol—commonly called pyro—and hydrochinon, are composed of the same chemical elements, carbon, oxygen, and hydrogen, the only difference in their composition being that a molecule of pyro contains one more atom of oxygen than the hydrochinon. The chemical formula for pyro is C6H6O3, and the chemical formula for hydrochinon is C6H6O2.
The chemical compounds employed in photography are used in the form of solutions. A solution is the liquid combination of a liquid and a solid. A simple solution is one in which the solid is entirely dissolved in the liquid, leaving the liquid transparent. A saturated solution is a liquid containing as much of the solid as can be dissolved in it and remain clear. In making saturated solutions, unless the exact proportions are known, add the solid to the liquid until there is a deposit of the solid at the bottom of the vessel containing the solution. The clear liquid can then be turned off carefully into a bottle.
A solid dissolves much more quickly if it is first powdered. If one has no mortar, put the solid inside a piece of muslin, lay it on a board or stone, and pound with a hammer. When powdered, put the cloth and powder both into a glass vessel, and turn the liquid over it. When the solid is dissolved, remove the cloth. Another way in which to dissolve a solid more rapidly than by mixing it with the liquid is to tie the powder in a cloth and suspend it in the liquid.
In making up a formula for developing or toning, etc., be exact in the measuring and weighing of the ingredients. Even a slight deviation from the rule sometimes changes the action of the chemicals.