THE EEL FAMILY. 385 



the temperature of the medium in which they exist ; and 

 this susceptibility may possibly account for their efforts to 

 reach the warm, brackish water of the tideways before the 

 colder months of the year, and also for the great numbers 

 of the young of other fish usually found in the same 

 situations. " It is a well-known law in chemistry 

 that when two fluids of different densities come in con- 

 tact, the temperature of the mixture is elevated for a 

 time in proportion to the difference in the density of 

 the two fluids, owing to the mutual penetration and con- 

 densation." Such a fusion is of course constantly going 

 on at the mouths of rivers debouching into the sea, and 

 the brackish mixture consequently maintains a tempera- 

 ture two degrees higher than either the pure salt or fresh 

 water. 



With the approach of winter (about October and No- 

 vember) the full-grown Eels descend the rivers in immense 

 shoals in search of these warmer waters : whether because 

 the process of spawning requires such a change, or simply 

 because it is agreeable to themselves, does not appear quite 

 certain. That they do descend, however, at least in the 

 great majority of cases, at that period, there can be no 

 doubt ; and that they will even overcome serious obstacles 

 in their course is equally well known. As to whether they 

 return to fresh waters again afterwards, authorities differ ; 

 but from the great size at which river-Eels are often taken, 

 one of three things is clear : either they do not all migrate, 

 or some of them (at least) return, or their growth-rate 



