94 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



pidity, analogous to that which is produced by its depression. Thus 

 the Helix pomatia (Edible Snail) has been found to become torpid and 

 motionless in water at 112; but to recover its energy when placed in 

 a colder situation. It would seem to be partly from this cause, but 

 partly also from the deprivation of moisture, that the hottest part of the 

 tropical year brings about a cessation of activity in many tribes of cold- 

 blooded animals, as complete as that which takes place during the winter 

 of temperate climates. 



141. The highest limit of temperature compatible with the life of 

 Fishes has not been certainly ascertained : and it appears probable that 

 there are considerable variations in this respect amongst different species. 

 Thus it is certain that there are some which are killed by immersion in 

 water at 104; whilst it is also certain that others cannot only exist, but 

 can find a congenial habitation, in water of 113, or even of 120; and 

 examples of the existence of Fishes in thermal springs of a much higher 

 temperature than this, have been put on record. Various fresh water 

 Mollusca have been found in thermal springs, the heat of which is from 

 100 to 145. Rotifera and other animalcules have been met with in 

 water at 112. Larvae of Tipulae have been found in hot springs of 

 205 ; and small black beetles, which died when placed in cold water, in 

 the hot sulphur baths of Albano. Entozoa inhabiting the bodies of 

 Mammalia and of Birds must of course be adapted to a constant tem- 

 perature of from 98 to 110 ; and they become torpid when exposed to 

 a cool atmosphere. These lowly organized animals seem more capable 

 of resisting the effects of extreme, heat, than any others ; at least if 

 we are to credit the statement, that the Entozoa inhabiting the intestines 

 of the Carp have been found alive, when the Fish was brought to table 

 after being boiled. In all such cases, it is to be remembered, that the 

 heat of the animal body must correspond with that of the fluid in which 

 it is immersed ; and we have here, therefore, evident proof of the com- 

 patibility of vital activity, in certain cases, with a very elevated tempe- 

 rature. Additional and more exact observations, however, are much 

 wanting on this subject. 



8. Of Electricity, as a Condition of Vital Activity. 



142. Much less is certainly known with respect to the ordinary influ- 

 ence of this agent, than in regard to either of the two preceding ; and 

 yet there can be little doubt, from the effects we observe when it is pow- 

 erfully applied, as well as from our knowledge of its connexion with all 

 Chemical phenomena, that it is in constant though imperceptible opera- 

 tion. Electricity differs from both Light and Heat in this respect ; 

 that no manifestation of it takes place so long as it is uniformly diffused, 

 or is in a state of equilibrium ; but in proportion as this equilibrium is 

 disturbed, by a change in the electric condition of one body, which is 

 prevented, by its partial or complete insulation, from communicating 

 itself to others, in that proportion is a force produced, which exerts 

 itself in various ways according to its degree. The mechanical effects 

 of a powerful charge, when passed through a substance that is a bad 

 conductor of Electricity, are well known ; on the other hand, the chemical 



