364 M. F. Plateau on the Aquatic Articulata. 



paper, and tlien washed five times with distilled water, until 

 the last washing-water scarcely produced a perceptible tur- 

 bidity with nitrate of silver. The nine Aselli were then placed 

 for the sixth time in pure distilled water (10 cubic centimetres) 

 and left therein for two hours. At the end of this time they 

 had recovered all their vivacity ; and the water in which they 

 had remained furnished, with nitrate of silver, a distinct ^re- 

 cipitate of chloride, soluble in ammonia. 



I have varied the conditions of these experiments, em- 

 ploying sometimes water containing less chloride of sodium 

 than sea-water, sometimes pure sea-Avater ; and I have always 

 arrived at results of the same kind. These seemed to me to 

 place it beyond doubt that certain aquatic Articulata absorb 

 chloride of sodium by the surface of the body ; but it was still 

 necessary to show that all the freshwater Articulata are not 

 in the same case, and that those in which there is no absorp- 

 tion are precisely those which are able to live with impunity 

 in sea-water. Now the experiments made upon Coleoptera, 

 Hemiptera, larvae of Agrion, &c. showed no excretion, and 

 consequently no absorption, of chloride of sodium. 



4. The injurious salts contained in sea-water are the chlo- 

 rides of sodium and magnesium ; the sulphates may be re- 

 garded as having no effect. 



I have arrived at this conclusion by examining successively 

 the action of solutions of chloride of sodium, of chloride of 

 magnesium, and of sulphate of magnesia, in such proportions 

 that in each case the weight of the single salt employed might 

 equal the sum of the weights of all the salts contained in sea- 

 water. The experiments were tried only with species in 

 which the presence of a delicate skin or of branchiaj rendered 

 a great absorption probable. 



The action of chloride of sodium proved to be sometimes 

 analogous to that of pure sea-water, and sometimes more 

 energetic. The action of chloride of magnesium is of the 

 same kind as that of chloride of sodium, or weaker, according 

 to the species ; this salt must therefore be regarded as inferior 

 to the preceding one in its injurious effects. The solution of 

 sulphate of magnesia produces no effect, or leads to death only 

 after a very long time. 



I have also been able to ascertain, by operating in accord- 

 ance with the process 3, that the larvae of insects and the 

 freshwater Crustacea experimented on only absorb a very 

 little of the chloride of magnesium, which may explain the 

 slowness of the* action of this salt in many cases. They gene- 

 rally do not absorb any trace of the sulphate. 



5. The difference of density which exists between fresh and 



