As a determination of heat production is of considerable interest the method will be given in some detail. Although the experiment sounds very simple, it is actually somewhat difficult to carry out. The attainment of temperature equilibrium between two solutions is very slow when one wishes to obtain them to within 0.001° C. of the same temperature. After many attempts, the following arrangement of apparatus (Fig. 33) was found most satisfactory. About 10 c.c. luciferin solution was placed in the inner tube (_D_) of a special non-silvered thermos bottle (_A_). About 1 c.c. of luciferase solution was placed in a very thin-walled glass tube (_E_) which was immersed in the luciferin solution and connected with a small motor so that it could be slowly but constantly rotated, thus stirring the solutions. Thermocouples (_L_ and _M_) of advance (.008 in)--copper (No. 30, _B_ and _S_, enamel insulated) wire were paraffined and placed in each tube and the copper wires connected through a copper double throw switch (_C_) with a Leeds and Northrup d'Arsonval wall galvanometer (No. 34637, silver strip suspension) of 35 ohms resistance and 310 megohms sensitivity. The constant temperature junctions (_N_) were placed in a large Dewar flask (_B_) filled with water at approximately the same temperature as the luciferin solution. One mm. galvanometer scale division represented 0.003° C. and the division readings could be estimated to tenths. By means of a glass rod (_F_) placed in the tube containing luciferase solution, this tube could be broken and the luciferase and luciferin solution mixed. [Illustration: FIG. 33.--Apparatus for determining heat production during luminescence of luciferin. A, special thermos tube. B, Dewar flask for constant temperature junctions. C, double throw switch. D, tube containing luciferin solution. E, tube containing luciferase solution. F, glass rod for breaking E. G, rubber stopper with groove, K, for pulley cord. H, cork closing thermos tube. J, brass sleeve in H allowing rotation of E. L, thermojunction in luciferase solution. M, thermojunction in luciferin solution. N, constant temperature junctions.] [Illustration: FIG. 34.--Curve showing temperature change when two tubes containing water at the same temperature are mixed. 0.1 galvanometer scale division = 0.003° C. Dots represent readings of thermocouple in tube D; crosses readings of thermocouple in tube E.]