The Nature of Animal Light

by E. Newton Harvey

Boyle also made many experiments to show that air was necessary for the life of animals and the germination

of seeds and showed that repeatedly respired air was unfit for further breathing. About the same time R. Hooke discovered the true meaning of respiratory movements and by forcing a blast of air continuously through the lungs with bellows, was able to keep animals alive. He concludes "that as the bare Motion of the Lungs, without fresh air, contributes nothing to the life of the Animal, he being found to survive as well as when they were not moved as when they were; so it was not the Subsiding or Movelessness of the Lungs that was the immediate cause of death, or the stopping of the circulation of the Blood through the Lungs, but the Want of a sufficient Supply of fresh Air." The cause of death on collapse of the lungs could not be better stated to-day. Thus combustion, respiration and luminescence of flesh or wood were early recognized as related phenomena.

Although the "gas sylvestre" (CO₂) of burning charcoal and fermentation of wine was known to van Helmont (1577-1644) and Mayow (1646-1679) in 1674 showed that "spiritus nitroærens" (oxygen) was responsible for the life of animals and for combustion, a century elapsed before the true significance of these gases became known. In the meantime the phlogiston theory of combustion had been developed, Black (1728-1799) in 1755 had rediscovered carbon dioxide ("fixed air") in the expired air and Priestley (1733-1804) and Scheele (1742-1786) had both rediscovered oxygen ("dephlogisticated air") in 1774. About the same time Lavoisier overthrew the phlogiston doctrine and showed that in the combustion of organic substances water and CO₂ are formed.

Later it was realized that this slow combustion did not take place in the lungs, or in the blood, but in the

tissues cells themselves and respiration in the chemical sense has come to mean this universal slow combustion in the cells of the body rather than the breathing movements of the lungs themselves. In anaerobic respiration, CO₂ is given off, but no oxygen absorbed. In aerobic respiration, oxygen is absorbed and CO₂ given off. In addition we know of many substances which oxidize by taking up oxygen without giving off CO₂. We have seen that oxygen must be absorbed for luminescence of animals and we may now inquire whether CO₂ is given off and the relation between respiration and light production.

To determine if CO₂ is given off during luminescence it is necessary to work with fairly pure luminous materials, obtained from luminous organisms. It is impossible to use the living organisms themselves as the CO₂ continually respired becomes a very disturbing factor. From Cypridina, a small crustacean, two materials soluble in water may be prepared (luciferin and luciferase), which will give a brilliant luminescence on mixing. It is possible to determine the H-ion concentration of the two solutions separately and of the mixture of the two after the luminescence has occurred.

If CO₂ is produced during luminescence the H-ion concentration of the luminous solution should increase. Measurements made electrometrically with the hydrogen electrode have failed to demonstrate any increase in acidity. The Ph of both solutions and of a mixture of the two is 9.04. This would indicate that CO₂ is not produced. As both luminous solutions contain proteins and the luminous substances themselves are probably proteins, which have a high buffer value, a method of this kind is none too sensitive. However, we can definitely state that not enough

CO₂ is produced to be detected and that this may be due to the buffer action of the luminous substances themselves. After all, unless luminescence is connected with respiration, we should hardly expect CO₂ to be produced.

Another method of testing CO₂ production is to measure the amount of heat produced during luminescence. Substances burned during respiration give off considerable heat, one gram of glucose to CO₂ and H₂O, as much as 4000 calories. We have seen in [Chapter III] that no infra-red radiation is produced in the light of the firefly. This does not mean, however, that no heat is produced by the reaction which produces the luminescence. A temperature change of a few thousandths or hundredths of a degree would evolve no measurable radiation. Coblentz (1912) first studied the problem of heat production in the firefly, using a thermocouple as the measuring instrument. He came to the conclusion that the temperature of the insect was slightly lower than the temperature of the air and that the luminous segments were slightly hotter than the non-luminous segments, whereas a dead firefly is of the same temperature as its surroundings. No definite increase or decrease in temperature could be established during the flash of the firefly. However, further work on the firefly is much to be desired.

The use of a living animal for such measurements introduces a possible source of error in that any contraction of the muscles of the animal will produce heat which may add to an increase or mask a decrease of temperature during luminescence. Utilization of extracts of luminous animals containing the luciferin and luciferase mentioned above avoids the complications due to muscular contraction. By bringing the solutions of luciferin and luciferase