For purposes of photography some substance must be used which will combine very quickly with the silver, and such a substance is found in silver nitrate. The chlorine set free by the action of light combines at once with the pure silver in the nitrate of silver. The chemical nature of the dark-colored substance produced by the action of light on the silver chloride is not yet fully determined, but most chemists agree that the silver chloride, when decomposed by light, produces silver sub-chloride and chlorine. (A sub-chloride is a chloride which contains more of the base than the acid. A molecule of silver chloride contains one atom of silver and one of chlorine, while a molecule of sub-chloride contains two atoms of silver and one atom of chlorine.)
This silver chloride is white, but passes through different shades of coloring, from a reddish-violet to a deep purplish-black, according to the length of time it is exposed to the light.
When paper coated with a sensitive silver solution is placed under a negative and exposed to the sun, the light reaches the paper through different degrees of thickness, or density, in the gelatine film. In the part of the negative which represents the sky the film is quite thick, while in the part which represents the deep shadows it is thin, sometimes being almost transparent. The part of the paper which is under the portion of the negative representing the sky is scarcely affected by the light, but in that part representing the shadows the light acts at once, and quickly decomposes the silver chloride. When the paper is taken from the printing-frame it contains different grades of the deposit formed by the action of light on the silver chloride.
Note.—The first article in the series of "Chemistry of Photography" was published in No. 867 (June 9). This article was on the chemical elements, and contained the following paragraph:
"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 atomic 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 the next number of the Round Table a list of the chemicals mentioned was given, but either through a typographical error or an error in copy, the weight of hydrogen was given as "11," and that of oxygen as "12." It should have been hydrogen "1," and oxygen "16." Those who read the first paper would of course perceive the mistake; but this correction is made for those who may not have seen the first article, or may have forgotten the explanation.
[CAPTAIN JACK AND THE CANNIBALS.]
"Well! well! well!" said old Captain Jack, as Bobbie and Tom appeared before him on the beach in front of the Ocean House. "You boys back again, eh? Why, do you know, I never expected to see ye again? For a fact I didn't."
"Why not?" asked Bobbie.
"Why not?" echoed the old seafarer, as he leaned back against the old wreck and laughed. "Why not? Why, I takes the town paper, I does, an' las' winter I seed a squib in the town paper as said that two hungry cannabiles had descended on New York city, an' et up the whole poppylation. Mebbe you didn't belong to the poppylation. Some folks don't join everything there is a-goin'. Wasn't ye et up?"
"How you do talk!" said Bobbie. "If we had been eaten up how should we be here?"