Although the "gas sylvestre" (CO_{2}) 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 CO_{2} 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, CO_{2} is given off, but no oxygen absorbed. In aerobic respiration, oxygen is absorbed and CO_{2} given off. In addition we know of many substances which oxidize by taking up oxygen without giving off CO_{2}. We have seen that oxygen must be absorbed for luminescence of animals and we may now inquire whether CO_{2} is given off and the relation between respiration and light production. To determine if CO_{2} 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 CO_{2} 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 CO_{2} 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 CO_{2} 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 CO_{2} 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 CO_{2} to be produced.