For instance, Ramsden in British Pat. No. '3640 (1897) provided a piece of paper, celluloid, metal or other suitable material which would be (1) coated with, or (2) formed with a hollow or recess containing a substance, mixture of material which would change in opacity at the temperature for the indication of which the appliance is intended to be used (see page 1, lines 24-38; page 5, lines 23-45