BOYCOTT : THE HABITATS OF FRESHWATER MOLLCSCA. 247 



(i). Physically our closed ponds have more considerable and sud- 

 den oscillations of temperature than larger and especially deeper 

 bodies of water. The temperature of a pond two or three feet deep 

 follows the air temperature pretty closely, even with a rising tempera- 

 ture, especially if the surface is stirred with a wind. There is no 

 stationary layer of cold water on the bottom as in deep lakes. In 

 the result such ponds are hit rather hard in short cold spells, and 

 with free exposure to the sun the temperature may rise to the neigh- 

 bourhood of 3o"C on a hot summer afternoon. But in makmg 

 similar observations on the waters of our river area I have been sur- 

 prised to find that the conditions are not as widely different as I had 

 anticipated. The river Colne itself rises and fails in temperature 

 rather less than our ponds, but it is not far behind, and the specially 

 prolific Dackwaters, at any rate about the edges where the character- 

 istic operculates and F. ant/iicu?H chiefly abound, behave practically 

 as does a small pond. I am inclined, therefore, to attach relatively 

 little importance to the temperature question m respect of the local 

 problem ; with deep lakes the state of afifairs is, of course, quite 

 dififerent. 



(2). A second physical, or rather physico-chemical consideration, 

 arises in muddiness. The factors which appear to be concerned in 

 this case are — («), the presence of clay ; {b\ mechanical disturbance 

 to suspend the clay in the water ; {c), the presence or absence of 

 sufficient salts in solution which will determine the agglutination and 

 precipitation of the suspended clay particles. Clay is practically 

 always present on the floor and sides of ponds ; the influence of 

 cattle or ducks illustrates well the results of mechanical disturbance ; 

 of the possible salts, those of calcium alone are present in any ordin- 

 ary water in sufficient quantities to be effective clarifying agents. 

 Hence hard water is clear, soft water is inclined to be muddy. Simi- 

 larly running water, which incidentally is mostly harder than still 

 water, will mechanically wash away clay put in suspension by some 

 casual disturbance ; in still water it has to fall to the bottom by natural 

 sedimentation, which may be, in soft water, an almost infinitely slow 

 process. The contrast, therefore, is very marked in this respect 

 oetween water which is both hard and running, such as our river, and 

 that which is at once soft and still, as in our ponds. The water of 

 the river and its appendages contains generally rather more than loo 

 milligrammes of calcium per litre (about 25 degrees of hardnessj; the 

 ponds vary from 10 to about 90, the average of the 72 which have 

 been examined being 45 milligrammes (about 11 degrees). The lakes 

 have quite soft water (25 milligrammes j, and the water is never clear; 

 though a fair volume of water is always passing through, there is con- 



