124 Effects produced by Fresh Water 



ebb tide might destroy them ; and it is only a casual circum- 

 stance that one of them is found in the usual place of digging 

 for bait. They must be sought for at the verge of low-water 

 mark, and they are only to be found in plenty, and of full 

 growth, during the neap tides. 



The common Lug (Zumbricus marinus) is, on the con- 

 trary, generally dug out of the sand at a considerable distance 

 from low-tide mark, and where it is left dry for many hours. 

 Showers of rain, therefore, we should suppose, can exercise no 

 deleterious influence on it ; and accordingly I found that some 

 lug, which I kept immersed for several hours in fresh water, 

 did not seem to be at all incommoded by it. 



That fresh water has a considerable influence on the vitality 

 of some marine plants is obvious from the following remarks. 

 "When we examine fresh specimens of i^ucus punctatus. Con- 

 ferva setacea, or U'Uo. dich6toma, we find that they possess a 

 very considerable degree of rigidity, or firmness. Now, I 

 have observed that these plants, after being immersed in fresh 

 water for a short time, lose their firmness, and become per- 

 fectly flaccid. They also change their colour in a material 

 degree especially the first two. These, when gathered fresh, 

 are of a garnet-red colour ; but, when immersed in fresh water, 

 the tint soon turns to orange. 



These, however, are not the only changes which take place. 

 I had long ago remarked that when recent specimens of 

 -Fucus punctatus or Conferva setacea are immersed in fresh 

 water for a few seconds, and then taken from it, they give out 

 a crackling or crepitating noise, like that made by fine salt 

 when thrown into a fire. I had observed, too, that during 

 the continuance of this crepitation. Conferva setacea (especially 

 if in fruit) projected minute globules of water, or some fluid, 

 to a distance of several inches. I last summer observed also 

 a weak crackling noise in LTlva dichotoma under similar cir- 

 cumstances. 



Now, in all these, the crepitation, when it ceases, which is 

 in a few seconds, may be renewed by again dipping it in the 

 fresh water, and then removing it; but at each successive 

 time the crackling is weaker, and it ceases altogether after a 

 third or fourth immersion. If the specimen, also, have been 

 in the water longer than a few minutes, the crackling is very 

 weak, or not perceived at all. The cause of it in Conferva 

 setacea I have clearly ascertained, and it may be easily seen, 

 with the help of a common pocket magnifier, or even by the 

 naked eye. 



I poured some fresh water on a common white plate to the 

 depth of about one twelfth part of an inch, and in this I put a 



