430 ANIMAL LUMINOSITY. 



sustain exposure to air saturated with vapour, even though it may not be 

 many degrees hotter than the body ; because the cooling act of evapo- 

 ration from the skin cannot then be carried on. 



769. The evolution of Light is a very interesting phenomenon, chiefly 

 witnessed among the lower animals, and usually supposed not to occur 

 in any class above Fishes. It is particularly remarkable among the 

 Radiata and inferior Mollusca. A large proportion of the Acalephce, or 

 Jelly-fish tribe, possess the property of luminousness in a greater or less 

 degree ; and it is to small animals of this class, which sometimes multi- 

 ply to an amazing extent, that the beautiful phenomenon of phosphores- 

 cence of the sea is chiefly due. In the midst of the soft diffused light 

 thus occasioned, brilliant stars, ribands, and globes of fire are frequently 

 seen; these appearances being due to the luminosity of the larger 

 species of the same tribe, or to that of other marine animals. Some of 

 the most remarkable examples of luminosity, in regard to the brilliancy 

 of the light emitted, occur in the class of Insects. Here the emission 

 is confined to one portion of the body, or to two or more isolated spots, 

 instead of being diffused over a larger surface ; and it is proportionally 

 increased in intensity. The phenomenon of Animal Luminousness 

 appears usually attributable to the formation of a peculiar secretion ; 

 which, in many instances, continues to shine after removal from the 

 animal, so long as it is exposed to the influence of oxygen: and it 

 seems not unreasonable to believe, that it depends upon a slow process 

 of combustion, analogous to that which takes place when phosphorus is 

 exposed to the air. There is a special provision in Insects, for convey- 

 ing a large supply of air through the peculiar substance, which is depo- 

 sited beneath the luminous spots ; and the power which Glow-worms, Fire- 

 flies, &c., possess, of suddenly extinguishing their light and as suddenly 

 renewing it, seems to depend upon their control over the air-aperture or 

 spiracle by which air is admitted, the stoppage of the supply of air 

 causing the immediate cessation of the luminousness, and its readmission 

 occasioning a renewal of the process on which it depends. It is proba- 

 ble, however, that in certain cases the luminosity is rather of an electrical 

 character. There are several of the smaller Annelida or marine Worms, 

 which are brilliantly luminous when irritated ; the luminosity having the 

 character, however, of a succession of sparks, rather than of a steady 

 glow. It appears from the experiments of M. Quatrefages, that this pecu- 

 liar luminosity is the especial attribute of the muscular system ; and that 

 it is produced with every act of muscular contraction in these animals. 



770. Although no such luminosity is commonly manifested in any of 

 the higher Yertebrata, or in Man, yet there are well-authenticated 

 cases, in which the phenomenon has presented itself in the living Human 

 subject,* luminous emanations from dead animal matter being of no 

 unfrequent occurrence. In most of these cases, however, the indivi- 

 duals exhibiting the luminosity had suffered from consumption, or some 

 other wasting disease, and were near the close of their lives at the 

 time ; so that it is probable that a decomposition of the tissues was 



* See an account of several cases of the Evolution of Light in the Living Human 

 Subject, by Sir Henry Marsh, M.D., M.R.I.A., &c. 



