The local temperature at the surface of sliding solids may reach a high value even under comparatively gentle conditions of sliding. With metals the surface temperature can be measured by using two different metals as a thermocouple; the temperature depends on the load, speed and thermal conductivity of the metals. With non-metals, which are poor thermal conductors, the high temperatures occur even more readily. The thermoelectric method cannot be used with non-conductors, but if one or both of the surfaces is of transparent material such as glass or quartz, the hot spots may be observed visually or recorded photographically. Experiments with surfaces of different melting-points show that the hot spots first become visible when the temperature is about 520-570 degrees C. Hot spots of this temperature may occur on glass or quartz when the load is about 1000 g. and the sliding speed as low as 1 or 2 ft./sec. These local surface temperatures play an important part in the chemical decomposition which accompanies the rubbing of solids. Experiments show that the detonation of a liquid explosive, such as nitroglycerine, by friction is due to the development of local high temperatures on the surface of the sliding solids. Under the conditions of these experiments the temperature of the hot spots necessary to cause the explosion of nitroglycerine is about 480 degrees C.