des Agens Physiques sur la Fie, 147 



seasons bi/ that which preceded the experiments. The modi- 

 fications of the seasons, therefore, appeared to influence the 

 cold-blooded animals used in the experhnents in this point of 

 view only. Accordingly we have this remarkable result, 

 that the animals lived twice as long in autumn as in the 

 summer preceding, when plunged in water of equal tempe- 

 rature. The seasons evidently influence their constitutions, 

 80 as to extend the duration of life independently of other 

 causes, that is, from summer to autumn. Dr. Edwards en- 

 deavoured to ascertain if it proceeds from atmospheric tempe- 

 rature, and he found that frogs lived in aerated water at ten 

 degrees, during November, from five or ten to eleven, and 

 even to forty hours, in some instances, the last term being 

 about double the duration of life in water of the same degree 

 in summer. This proves the remarkable dependence of the 

 frog's life under water, and the temperature of the month 

 preceding. Two curious facts are thus developed by ex- 

 periments instituted at difi*erent seasons. First, the influence 

 of the temperature • of the water in which the animals were 

 placed ; and secondly, the influence of the temperature of the 

 air during certain periods preceding the experiments, for 

 in autumn the duration of life was about double that of 

 summer, and in winter he found the term to equal autumn, 

 the temperature of the air being in each comparative expe- 

 riment artificially raised to the same degree. 



It appears from the foregoing experiments that frogs, 

 toads, and salamanders, exist in water according to its low- 

 ness of temperature, and that their lives are prolonged bi/ 

 the temperatvre which precedes the experiment being lowered. 

 It then becomes a question, what are the limits of this influ- 

 ence ? This is to be ascertained by observing the greatest 

 duration of life among animals deprived of external air by 

 submersion in water ; and noticing at the same time all the 

 favourable circumstances dependent on the concurrent tem- 

 perature in prolonging life among the cold-blooded animals, 



A point relative to the natural history of frogs first pre- 

 sents itself to our notice. Spallanzani is of opinion that frogs 

 do not pass the winter under water, but retire in October 

 from their native rivers into moist sands, in which they make 

 openings to breathe the air through, called by the Italian 

 fishermen il respiro delta rana. 



M. Bose, and other Frentrh naturalists, found that frogs 

 retire from October to spring into water, but they give us no 

 direct proof that they constantly remain submersed. The 

 presence of the observer may alarm the frogs, and thus pre- 



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