Important: Safety Warning
This page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
Collodion-Process
a process in photography invented by Archer, in 1851. An iodized collodion is made by impregnating a solution of gun-cotton in ether, with a small quantity of iodide of potassium or cadmium. A film of the iodized collodion is spread on the glass, which is then immersed in a solution of nitrate of silver. The image is taken in the camera, developed by a weak solution of pyrogallic acid and acetic acid, or a solution of protosulphate of iron.
Excess of iodide of silver is removed by hyposulphite of soda or cyanide of potassium. This gives a negative. A positive is obtained by laying the negative on prepared paper and exposing them to light.
COLLOIDS. The first extensive investigation of colloids was made by Thomas Graham in the years 1860-1864.
He found that certain substances formed what were, apparently, solutions, in that they would pass, unchanged, through filter-paper, but that they differed from solutions of crystalloids, such as sugar or salt, in that they would not dialyze. See DIALYSIS. These substances he called colloids. Well-known examples are starch, glue, dextrin, albumen, caramel, and tannins. All living matter is, for the most part, built up of colloids. Recent investigations have tended to show that it is possible to bring any substance into a colloidal condition, and colloidal solutions of metals and their compounds have been prepared. The word "colloid" is now used, therefore, to describe a state of matter rather than substances. a particular class of