A large number of proteid substances, when in solution, are coagulable by heat. As the temperature of such a fluid is raised, faint opalescence at first appears, and then, at a higher temperature, masses (flocculi) of albumen separate out, in most cases, suddenly, from the fluid. It is generally held that each coagulable albumen is so affected at a definite temperature peculiar to itself; thus, egg albumen is said to become opalescent at 60° C, and to separate out in flocculi at 63° C. Unfortunately, hardly two observers agree as to the exact temperature at which opalescence and coagulation occur; thus, keeping to the example, egg albumen, Wurtz puts the coagulation point at 73° C, and Henrijean at 60° to 61° C.