80 CHARACE^5. 



Various attempts have been made to determine the cause 

 of the remarkable circulation in Chara and Nitclla. The in- 

 sufficiency for this purpose of the well-known laws of endos- 

 mosis and exosmosis has been ingeniously shown by Mr. Slack 

 in a note inserted in the Second Part of vol. xlix. of the 

 " Transactions of the Society of Arts," that lamented inquirer 

 having ascertained that the circulation continued in all plants 

 in which it has been observed when the portion was immersed 

 in oil or mucilage, and in strong saline solutions, and even 

 when not in contact with fluid of any sort, but surrounded 

 only by the dry atmosphere. 



In a letter addressed to the Royal Academy of Sciences, 

 of Paris, in the sitting of November 23d, 1829, by M. Du- 

 trochet, some highly interesting remarks occur relative to the 

 circulation of Chara. The discovery was made very many 

 years ago by Count Rumford, that water placed in vertical 

 tubes circulates. In order that the circulation of so transparent 

 a fluid may be the better appreciated it is requisite to suspend 

 in it some molecular substance, the best for which purpose M. 

 Dutrochet has found to be a few drops of milk, the specific 

 gravity of which is nearly equal to that of the water. Many 

 other substances will circulate in water, such as very fine 

 sawdust, amber reduced to powder, and rasped cork ; but 

 these being of greater specific gravity than the element in 

 which they are suspended, sooner or later subside to the 

 bottom of the tube. 



M. de Bailif illustrated, by means of an apparatus con- 

 structed on this principle, the circulation which takes place 

 in the cells of Chara, &c. M. Dutrochet, however, in the 

 letter above cited, examines this curious physical phenomenon 

 more closely. He found that the efficient cause of the cir- 

 culation was caloric, that the rapidity of the motion depended 

 on the amount of caloric to which the tube and its contents 

 were subject, and that the direction of the current was go- 

 verned by the same agent, the ascending stream being placed 

 always on the side of the tube which received the most heat. 

 M. Dutrochet observed, moreover, that light exerted a power- 

 ful influence on the circulation, which influence was attri- 



