5. When spontaneous light is extinguished by some bodies or substances, it is not lost, but may be again revived in its former splendour, and that by the most simple means.
These means are in general the greater or less degrees of impregnation of the saline menstrua, which alternately extinguish and revive the light at the option of the operator.
6. Spontaneous light is not accompanied with any degree of sensible heat, to be discovered by a thermometer.
7. Cold extinguishes spontaneous light, but not permanently, as the substances mentioned in the third article; since here the light could be revived in its full splendour, by exposing the substance to a moderate degree of temperature.
8. The eighth section treats of the effects of heat on light when in a state of spontaneous union. We gather from the experiments here described, that in every substance there is a certain point of temperature at which it acquires its maximum of lustre; that this varies considerably in various substances,—the fishes, rotten wood, and aqueous solutions, becoming dark at a temperature of between 96° and 110°; while glow-worms retain their lustre until exposed to the degree of boiling water. Common water impregnated with light, when by mere time and rest, without any considerable change of temperature, it had become obscure, was soon rendered luminous when gradually warmed by small and successive additions of warm water; but no sooner was boiling water added in any considerable quantity, than the luminous appearance vanished, and was altogether extinguished.
It was here observed, that if heat be applied to the bottom of a tube filled with illuminated water which has been some time at rest, the light will descend in luminous streams from the top of the tube to the bottom, and be gradually extinguished.
9. In the ninth and last section we find a set of experiments on the effects of spontaneous light when applied to the human body. As to the living body, it was found that on touching the luminous matter of fishes, the light adhered copiously to the fingers and different parts of the hands, remained very lucid for some time, but then gradually disappeared; whereas the same kind of matter being applied to pieces of wood, stone, and the like, of the same temperature as the laboratory, continued luminous on these substances for many hours. It might hence be inferred that the living body ultimately absorbs the light; but other experiments seem to contradict this inference, it having been observed that a piece of shining wood being placed on the palm of the hand, and a glow—worm being breathed upon repeatedly, were both rendered more luminous and this for a considerable time.
As to the animal fluids; the crassamentum of the blood of healthy persons, and also of some labouring under inflammatory diseases, received indeed the light of a herring to a certain degree, but did not retain it long: and when this coagulum had been kept for some time,
