The effect of temperature on _Cypridina_ luminescence also bears out the preceding conclusions. For a given mixture of luciferin and luciferase the light becomes more intense with increasing temperature up to a definite optimum and then diminishes in intensity. The diminution in intensity above the optimum is due to a reversible change in the luciferase so that its active ma.s.s diminishes. This change becomes irreversible in the neighborhood of 70 (depending on various conditions), where coagulation of luciferase occurs. Light will appear at 0 but it is far less intense than light at higher temperatures and it is more yellow in color. The light of optimum temperatures is quite blue. The weaker light at temperatures above the optimum is also more yellow in color. I believe this difference in color is a function of the slowed reaction velocity, for a mixture of luciferin and luciferase which gives a bluish luminescence at room temperature, will give a weaker and yellowish luminescence if diluted with water. Dilution with water will slow the reaction velocity. If the difference in color were not real but due to change in color sensitivity of the eye with different intensities of such relatively weak light (Purkinje phenomenon), the weaker light should appear more blue. As the weaker light appears more yellow, I therefore believe the color difference is actual and not subjective.

A minimum, optimum, and maximum temperature for luminescence is observed in all luminous organisms. The minimum is usually very low. Luminous bacteria will still light at -11.5 C. The power to luminesce under ordinary conditions is not destroyed by exposure to liquid air, for, on raising the temperature, light again appears (Macfayden, 1900, 1902).

Almost all organisms will luminesce at 0 C., and the luminescence minimum probably represents the point at which complete freezing of the luminous solution occurs. It is very low with bacteria because they are solutions in capillary s.p.a.ces of very small size, a condition tending to lower the freezing point.

The luminescence maximum represents the point at which luciferase is reversibly changed so as to be no longer active. If the temperature is again lowered the luciferase again becomes active and light reappears.

Some degrees above this, and in all forms well below the boiling point, luciferase is coagulated and destroyed. As the coagulation point of proteins depends on many factors, such as time of heating, salt content, acidity, etc., so the luciferases of different animals coagulate at different temperatures depending on these conditions. Some of the more reliable observations on these critical temperatures are collected in Table 14.

TABLE 14

_Temperature Limits of Luminescence for Luminous Organism_

-------------------------+---------------------+-------+-------+-------+ Organism

Author and date

Minimum

Optimum

Maximum

-------------------------+---------------------+-------+-------+-------+ Pseudomonas javanica

Eijkman, 1892

-20

25-33

45

Bacterium phosph.o.r.escens

Lehmann, 1889

-12

...

39.5

Bacterium phosph.o.r.eum

Molish, 1904, book

-5

16-18

28

Light bacteria

Tarchanoff, 1902

-7

15-25

37

Light bacteria

Harvey, E. N., 1913

-11.5

15-20

38

Mycelium X

Molish, 1904

...

15-25

36

Lampyrids

Macaire, 1821

-10

33

46-50

Pyrophorus noctilucus

Dubois, 1886

...

20-25

47

Photuris pennsylvanica

Lund, 1911

...

...

50

Luciola viticollis

Harvey, E. B., 1915

<0 ...="" 42="" cypridina="" hilgendorfii="" harvey,="" e.="" n.,="" 1915=""><0 ...="" 52-54="" cyclopina="" gracilis="" lund,="" 1911="" ...="" ...="" 50="" phylirrhoe="" bucephalum="" panceri,="" 1872="" 44="" ...="" 61="" pyrosoma="" panceri,="" 1872=""><0 ...="" 60="" mnemiopsis="" leidyi="" peters,="" 1905="" 9="" 21="" 37="" noctiluca="" miliaris="" quatref.a.ges,="" 1850="" 1="" ...="" 40="" noctiluca="" miliaris="" harvey,="" e.="" b.,="" 1917=""><0 ...="" 48="" cavernularia="" haberi="" harvey,="" e.="" n.,="" 1915=""><0 ...="" 52="" watasenia="" scintillans="" shoji,="" r,="" 1919="" ...="" 16-31="" 49="" -------------------------+---------------------+-------+-------+-------+="">

We are thus led to the conclusion that intensity of luminescence is dependent on the velocity of oxidation of luciferin and that with lowered reaction velocity the spectral composition of the light changes.

The maximum emission shifts toward the yellow. I believe, however, that in _Cypridina_ also, the luminescence intensity depends not only on reaction velocity but on the particular manner in which luciferin is oxidized. _Cypridina_ luciferin will luminesce only in presence of _Cypridina_ luciferase and no light can be obtained from _Cypridina_ luciferin and a host of different oxidizers (with or without H_{2}O_{2}) such as are able to oxidize pyrogallol. Luciferin will also oxidize in the air spontaneously but no light is produced. It is easy to show that this spontaneous oxidation may be much more rapid than an oxidation with luciferase and yet light appears only in presence of the latter. If a concentrated solution of luciferin is kept near the boiling point it will be completely oxidized to oxyluciferin in four or five minutes. No light appears if air or even if pure oxygen is bubbled through it. The same solution kept at 20 with a small amount of luciferase will luminesce continuously and not be completely oxidized to oxyluciferin in a half hour. We can, however, cause the luciferin to oxidize as rapidly at 20 by adding concentrated luciferase as does the luciferin near the boiling point without luciferase. A bright light is produced in the former case, none in the latter case. The oxyluciferin formed from spontaneous oxidation of luciferin appears to be the same as that formed with luciferase present. Both give luciferin again on reduction. Perhaps the reaction takes place in two stages, similar to those supposed to occur in other enzyme actions:

luciferin + luciferase = luciferinluciferase luciferinluciferase + O (or minus H_{2}) = oxyluciferin + luciferase.

We may then a.s.sume as a tentative hypothesis that luminescence only occurs during oxidation (addition of O or removal of H) of the luciferinluciferase compound.

We have just seen that the effect of cooling a _Cypridina_ extract containing luciferin and luciferase and luminescing with a bluish light, is to reduce the intensity and change the shade toward the yellow.

Velocity of oxidation must be lowered and with the same concentration of luciferase lowered velocity means more light of the longer wave-lengths.

A very instructive experiment on color of the light can be carried out with animals having different colored lights and so closely related that their luciferins and luciferases will interact with each other. Such a case is presented by the American fireflies, _Photinus_ and _Photuris_.

_Photinus_ emits an orange light, while _Photuris_ emits a greenish yellow light. The difference in color is especially noticeable when the luminous organs of the two forms are ground up in separate mortars. As shown by Coblentz, the difference in color is real, the spectrum of _Photinus_ extending farther into the red than that of _Photuris_ (see Fig. 8). We can easily prepare luciferin and luciferase from the two fireflies and make the following mixtures:

_Photinus_ luciferin _Photinus_ luciferase = reddish light.

_Photinus_ luciferin _Photuris_ luciferase = yellowish light.

_Photuris_ luciferin _Photuris_ luciferase = yellowish light.

_Photuris_ luciferin _Photinus_ luciferase = reddish light.

Thus the color of the light in these "crosses" is that characteristic of the animal supplying the luciferase. To bring this fact in line with what we have already said regarding reaction velocity and luminescence, we must believe that the _Photinus_ luciferase oxidizes at a slower rate than the _Photuris_ luciferase. In this connection it is of interest to recall that the _Photuris_ light as emitted by the insect becomes reddish at high temperatures, or if the insect is plunged into alcohol, both conditions which bring about partial coagulation of the luciferase and reduce its active ma.s.s.

BIBLIOGRAPHY

A few of the enormous number of papers on luminescence are included in the list below. The attempt is made to list only those dealing with the structure, chemistry or physiology of luminous animals and the physical nature of their light, together with a small number of general interest.

More complete works on light and luminescence come first and original articles follow. Authors' names are arranged alphabetically, their papers chronologically. A fairly complete list of literature covering the whole field of Bioluminescence is given by Mangold, 1910. The 1913 paper of Dubois gives a bibliography of his own contributions up to this date so that only those papers to which special reference is made are included below.

BOOKS AND GENERAL WORKS

BECQUEREL, E.: 1867, La Lumiere.

DAHLGREN, U.: 1915, The Production of Light by Animals. _Jour. Franklin Inst._, vols. 180 to date.

DUBOIS, R.: 1914, La Vie et La Lumiere. Alcan, Paris.

GADEAN DE KERVILLE, H.: 1890, Les Vegetaux et les Animaux Lumineux.

Paris.

HARVEY, E. N.: 1917, The Chemistry of Light Production in Luminous Organisms. Carnegie Inst., Wash., Pub. No. 251, pages 171-234.

HEINRICH, Pl.: 1811-1820, Die Phosph.o.r.escenz der Korper, etc. Nurnburg.

HOUSTOUN, R. A.: 1915, A Treatise on Light. London.

KAYSER, H.: 1908, Handbuch der Spectroscopie. Vols. ii and iv. Leipzig.

MANGOLD, E.: 1910, Die Produktion von Licht. Hans Winterstein's Handbuch der vergleichende Physiologie, vol. iii, second half, pp. 225-392. Jena.

MOLISH, H.: 1904 and 1912, Leuchtende Pflanzen. Eine physiologische Studie. Jena.

NUTTING, P. G.: 1912, Outlines of Applied Optics. Philadelphia.

PHIPSON, T. L.: 1870, Phosph.o.r.escence. L. Reeve and Co. London. 210 pages.

SHEPARD, S. E., 1914, Photochemistry. Longmans, Green and Co.

ORIGINAL PAPERS

ABEGG, R., and AUERBACH, F.: 1907, Handbuch der anorganischen Chemie.

Leipzig, vol. iii, pt. 3, p. 376.

AGa.s.sIZ, A.: 1874, Embryology of the Ctenophorae. _Mem. Am. Ac. Arts and Science_, x, p. 371.

ALLMAN, G. I.: 1862, Note on the Phosph.o.r.escence of Beroe. _Proc. Roy.

Soc. Edinb._, iv, 518.

AUBERT ET DUBOIS, R.: 1884, Sur les proprietes de la lumiere des pyroph.o.r.es. _Comp. rend. Acad. des Sc._, vol. xcix, p. 477.

BACH, A.: 1911-1913, Zur Kenntnis der Reduktionsfermente. _Biochem.

Zeit_, x.x.xi, 443; x.x.xiii, 282; x.x.xviii, 154; lii, 412-422.

BAKER, J.: 1743-1753, The Microscope Made Easy and Employment for the Microscope.

BALLNER, F.: 1907, Ueber das Verhalten von Leuchtbacterien bei der Einwirkung von Agglutinationsserum und anaesthesierenden chemischen Agentien, etc. _Centralb. f. Bact._, 2 abt., xix, 572.

BANCROFT, W. D.: 1913, The Chemical Production of Light. _Journ. Frank.

Inst._, clxxv, 129.