CHARACEJE. 81 



butable to the caloric which accompanied its rays, and also that 

 mineral substances in solution accelerated its speed. The 

 caloric and the mineral substances in solution were supposed 

 to exert this power by causing the greater separation of the 

 molecules of the water, and thus increasing their mobility. 

 Pressure was found to have a contrary influence, and to re- 

 tard the circulation by, it was presumed, occasioning the 

 closer approximation of the molecules, and so impeding their 

 mobility. Viscid organic matter, such as gum, produced the 

 same effect, and for a similar reason. 



The causes not favourable to the sustenance of this physical 

 circulation, therefore, are, the absence of caloric, or cold, and 

 of light, upon which it follows that the motion ought not to 

 take place during the night, pressure, and the solution of 

 viscid substances in the liquid ; all of which act in the same 

 way, viz. by impeding the facility of motion between the 

 aqueous molecules. On the contrary, the favourable causes 

 are all those things which act in an opposite way, by increas- 

 ing that facility. 



A very curious fact is related by Dutrochet in reference 

 to the circulation of aqueous fluid in tubes. It is this : that 

 if a drop or two of acid, of alkaline solution, or a solution 

 somewhat saline be added to the water contained in a tube, 

 and which has had milk mixed with it, these substances, more 

 weighty than water, precipitate themselves, and become dis- 

 solved. " This solution being achieved, the water is no 

 longer susceptible of presenting the circulatory movement 

 by means of simple diffused light ; it only presents this move- 

 ment at its superior part, and this only when the tube is 

 exposed to the direct light of the sun, the continued action 

 of which on the tube for many hours can scarcely penetrate 

 the circulation to the depth of an inch in that water, whose 

 molecules have acquired a very extraordinary molecular fixity. 

 I consider this fixity as the result of a regular disposition of 

 the molecules of the liquid. Indeed, when one agitates this 

 liquid endowed with molecular fixity, it becomes immediately 

 susceptible of circulation under the influence of the simple 



G 



