a much lower spontaneous reaction velocity than it seems probable to attain.
It is perhaps legitimate in a book on infrared photography to mention color changes that some materials undergo on being heated, with or without preliminary exposure to light. It is well known that some chemical compounds, for instance, mercuric iodide, change color on being heated to a certain temperature, and revert to their original color on cooling. A double compound of the iodides of mercury and copper, in fact, was used as a "thermoscope," to indicate temperatures by its change in color. Some compounds will form a latent image on exposure, particularly to ultraviolet, which is developed to a visible image on heating. A number of these have been described by Sheppard and Vanselow,62 particularly silver and mercurous oxalates, silver and lead formates, lead thiocyanate, the orthoarsenite of silver and other metals, and a variety of other mercury compounds, especially mercurous azide, thiocyanate, and chloride hydrazine. These are called photothermographie compounds.
Sheppard and Vanselow62 described another range of compounds which they called "thermographie," and which change or develop a color when heated alone. Oxalates again are used, but they are sensitized by a polyhydroxy alcohol such as glycerin, propylene glycol, and glucose. The thermographie reaction may be catalyzed by certain additions. It is possible that some of these mixtures could be used for photographic thermometry utilizing their direct darkening or coloring under heat.
1. BALL J. A., "Infra-Red Photography in Motion Picture Work," Trans. Soc. Motion Picture Engrs., 1925, no. 22, 21-4.
2. BALY E. C. C., Spectroscopy, 3d ed., Longmans, Green, London, 1927, vol. II, ch. V, "The Photography of the Spectrum."
3. BARTH W., and DUERR H. H., assigned to Agfa Ansco Corp., U. S. pat. 2,134,546.
4. BENNES C. D., assigned to Technicolor Motion Picture Corp., U. S. pat. 2,220,882; Fr. pat. 863,696.