914 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
phosphorus or not. It is easy to answer in the negative, that the 
light has nothing to do with the element phosphorus, which is too 
poisonous to be found in living cells. On the other hand, the light 
has a very great. resemblance to the luminescence of phosphorus. 
In the first place, it is an oxidation, and if we remove the oxygen 
from any luminous animal, the light will disappear completely, and 
if we again readmit oxygen, the light will return. This is a very 
interesting experiment, and a very old one. In fact, it is one of the 
first experiments that was ever made on luminous forms, by Robert 
Boyle, in 1667. 
Boyle at that time was experimenting with his air pump, and, 
among other things, he placed a little piece of “shining wood” or 
phosphorescent wood under the receiver of his air pump. He found 
that when he exhausted the air, the light disappeared, and when he 
readmitted air, the light returned. Of course, he did not know that 
it was oxygen in the air which was responsible for the effect, but 
nevertheless I think we can credit him with the discovery that the 
luminescence of living things requires the presence of free oxygen. 
The second chemical fact is also rather an old one. It was dis- 
covered by Spallanzani, an Italian, in 1794, that all luminescence re- 
quired water, and he showed that he could take any luminescent ani- 
mal, and dry it and the light would disappear, but that if he kept this 
dried material and at some later time moistened it again, the light 
would reappear. So, like the experiment with oxygen, we have 
another perfectly reversible process, showing that luminous animals 
require water in order to luminesce. 
This experiment also shows that luminescence is not a function of 
living cells in the same sense that the contraction of a muscle or 
propagation of a nerve impulse is a function of living cells. If a 
muscle is dried quickly, its form or constituents are not changed, but 
if put in water again, although it will look like the original muscle, 
no contraction will result on stimulation. The muscle has lost its 
contracting power by drying, and a nerve also loses its conducting 
power after drying. Therefore, we have in these tissues loss of a 
living function, but we do not observe loss of the power of lumines- 
ence on drying the luminescent organ of an animal. 
Since water and oxygen are necessary, it is likely that some mate- 
rial produced by the cells of the animal is oxidized, and this material 
is called, to use a general term, the photogen, but to use a more spe- 
cific term, it is called luciferin. 
In fact, not only one material, but two materials are found to be 
necessary in order to get light, in addition to water and oxygen. 
This is the third discovery in connection with the chemistry of 
luminescence, made by a Frenchman, Dubois, in 1887. He found that 
a luminous extract of an animal could be separated into two parts, 
