CYPRIDINA LUCIFERASE.--Luciferase, on the other hand, has _all_ the properties of a complex protein. It will not dialyze through collodion or parchment membranes, is soluble only in aqueous solvents, and hence precipitated by alcohol and acetone, digested by proteolytic enzymes, readily changed by contact with dilute acid and alkali and irreversibly coagulated on boiling. It is completely salted out of solution by saturation with (NH_{4})_{2}SO_{4} and nearly completely precipitated by the alkaloidal reagents. Its other properties are given in Table 8. Taken together, they point to the group of albumins as the class of proteins with which luciferase most closely agrees. If luciferase is not a protein it is so closely bound up with protein that it cannot be separated. This is characteristic of many enzymes and luciferase is also an enzyme. We can determine this by finding out whether luciferase will accelerate the oxidation of a large amount of luciferin, for such is the test of a catalytic substance. If we take 1 c.c. of a dilute solution of luciferase (1 _Cypridina_ to 50 c.c. water) and add to it successive 1 c.c. portions of concentrated luciferin (1 _Cypridina_ to 2 c.c. solution) as soon as the light from the preceding addition has disappeared, after four 1 c.c. additions, no more light is produced. The luciferase is therefore used up and cannot oxidize more than a certain quantity of luciferin. In this experiment, however, we added a concentration of luciferin from one _Cypridina_ 100 times that of the luciferase from one _Cypridina_, i.e., four additions each 25 times as concentrated. We have, of course, no way of telling what the absolute amount (in milligrams) of luciferin or luciferase is in a single _Cypridina_, but we do know that the luciferase from one _Cypridina_ cannot oxidize luciferin from more than 100 Cypridinas. If the ratio of luciferin to luciferase in a single animal is 100:1, it would mean that luciferase could oxidize 10,000 times its weight of luciferin. A large excess of luciferin but not an indefinite quantity can be oxidized by luciferase, and I believe this is sufficient justification for considering luciferase an enzyme, although it is not an ideal example of an organic catalyzer. Quite a number of enzymes are known to be diminished during the course of the reaction they accelerate or to be poisoned by their reaction products. Enzyme reactions inhibited by the formation of reaction products again proceed if these are removed or diluted. However, light does not again appear in a mixture of weak luciferase with excess of luciferin upon dilution with water, so that the luciferase cannot have been merely inhibited by some reaction product but must have been actually used up during the reaction. It should be noted in passing that the peroxidases, ordinarily spoken of as oxidizing enzymes, are used up in the reaction and can only oxidize limited amounts of oxidizable substances, a quantity almost in