68 SHRUBSOLE : EROSION OF CERTAIN FRESHWATER SHELLS. 



shell, and that the slightest, almost imperceptible, inequality in 

 the surface would result in an electrolytic action of the water, 

 which would gradually dissolve portions of the shells." With 

 all due respect to the physicist, we fail to see that he helps us 

 in any way to solve the problem. The want of homogeneity in 

 the shell will hardly apply, since no erosive action is set up 

 until the shell is removed into the water from another river, 

 and then it will be seen that it is not every water that will pro- 

 duce the erosion, but only waters from certain districts. 



From the outset of my inquiry I was led to think that it 

 might arise from some peculiarity in the water, for I found that 

 the shells of other mollusks in company with the Planorbes 

 were similarly acted upon. There was further the fact, that 

 during the time the Planorbes were living in the Trent Canal 

 the shell was entire, while three months'' existence in water taken 

 from the River Dee produced the appearance I have described. 

 In other words the mollusk could mature and maintain a shell 

 in the Trent Canal, but not in the water from the Dee. This 

 suggested that the presence or absence either in one or other 

 of the waters, of some element might set up the action I 

 have described. The solution of the question, it seemed 

 to me, must be sought for in the water. Obviously the next 

 point was to analyse the waters, and ascertain their leading 

 constituents. A little reflection suggested that this inquiry 

 might be narrowed down to one or two items usually present in 

 water, viz., calcic carbonate and carbonic acid. My reason for 

 limiting it thus, was that as it is well known the shelly structure 

 of the moUusca is mainly built up of calcic carbonate, a sub- 

 stance which is largely soluble in water in which carbonic acid 

 gas is dissolved. The latter as a matter of fact is largely 

 present in all running streams, and is undoubtedly the active 

 agent in dissolving out of various mineral substances in nature 

 the calcic carbonate which is present, and forming with such 

 a soluble compound. This is so extensively done, that is, that 

 water so charged with carbonic acid gas is constantly coming 



J.C, v., July, 1SS6. 



