Q. 3906. A Thermometric Paint.

Manufacturer and builder 3, 1887

I clip the following item from one of the technical papers, and would be glad to have your opinion of its utility: "Engineering, in a recent issue, refers to a red paint, recently brought out, which possesses the property of changing its color as it gets hotter until at a certain temperature it becomes very dark. On being allowed to cool, it returns to its original hue. The property is said not to disappear with age, and, if so, the paint may be useful in some ways, at least as a rough-and-ready indicator of temperature. For example, the temperature of bearings, or other parts of machinery, of conducting wires, and so on, might be indicated by its means, especially if a standard of comparison or scale of colors showing the equivalent of temperatures were provided with the paint to test it by. It was especially for this purpose that a similar pigment was placed before the public some years since."
— A. J. P., Trenton, N. J.

Answer. As nothing is said in this clipping about the chemical nature of this compound, it is impossible to do more than guess at its identity. It is more than probable, however, that the red paint here referred to is identical with the double iodide of mercury and copper, first discovered and described by a German chemist, Meusel, in 1870 or 1871, and which possesses to a remarkable degree the property of rapidly darkening under the influence of a moderate heat (150° to 300° Fah.), and of regaining its original red color on cooling. This substance was proposed for exactly the same purposes as suggested in the clipping from Engle. eering—namely, as an application to bearings and moving parts of machinery to indicate their undue heating—by Profs. Mayer and Barker, and a United States patent for this application was granted to them about the date named above. It does not appear, however, to have come into use. The English reference appears to us to be a clear case of reinvention, the last inventor having been anticipated by about sixteen years.