irhoe bucephala_, is also
fully explained, ammonia being one of those alkaline substances which
are so directly favorable to the exhibition of the phenomenon.

Nor is it difficult to account for the control which in some cases
insects appear to have over the luminosity of the phosphorescent
organs, exhibiting and withdrawing the light at will. It is not
necessary to suppose that this is an immediate effect, a conversion of
nerve force into light, and a withdrawal of that force. The action of
the creature's will may be merely in maintaining or destroying the
conditions under which the light is manifested. It may, for example,
have the power of withdrawing the supply of oxygen, and this
supposition receives some countenance from the observation cited from
Kirby and Spence on the two captured glow-worms, one of which
withdrew its light, while the other kept it shining, but while doing
so had the posterior extremity of the abdomen in constant motion. But
the animal may also have the power in another way of affecting the
chemical conditions of the phenomenon. It may, for example, have the
power of increasing or diminishing by some nervous influence the
supply of the necessary alkaline ingredient.

But if animal phosphorescence is really due to a process of slow
oxidation, there is one singular circumstance to be noted in
connection with it. Oxidation is a process that is normally
accompanied by the development of heat. Even where no light is
produced an increase of temperature regularly takes place when
substances are oxidized. We ought, then, to expect such a rise of
temperature when light is emitted by the phosphorescent organs of
animals. But the most careful observations have shown that nothing of
the kind can be detected. It was with a view to test this that Panceri
dissected out the luminous organs of so many specimens of Pholas. He
selected this mollusk because it was so abundant in the neighborhood
of Naples, where, his experiments were made; and in making his
experiments he made use of a thermopile, an apparatus by which, with
the aid of electricity, much smaller quantities of heat can be
indicated than by means of the most delicate thermometer. The organs
remained luminous long after they were extracted, but no rise in
temperature whatever could be found to accompany the luminosity. Many
experiments upon different animals were made with similar negative
results by means of the thermometer.

The only explanation