150 Intelligence and Miscellaneous Articles. 



culiar movements. In benzoic acid the principal character of the 

 movement is a rotation round a point sometimes lying within, and 

 sometimes without the crystals. In succinic acid this rotation 

 alternates with straight movements taking place by jerks, like the 

 course of the water-spiders on sunny days in summer. 



The power with which many bodies sprinkled upon the water 

 (such as semen Lycopodii) are carried along by the moving crystals, 

 is very remarkable, and still more so the circumstance that all motion 

 ceases almost instantly, when one finger of the bare hand is immersed 

 in the water. 



If a portion of the water in which the crystals have been brought 

 to a standstill be poured away, their motion commences with re- 

 newed force, and lasts, if no further disturbance occurs, until the 

 crystals are dissolved. That this movement is a consequence of 

 solution by the water, and produced directly by the one-sided attack 

 of the solvent upon the crystals, may be concluded a priori ; but 

 that the behaviour of these crystals should be so modified by touching 

 the water, that as the solution continues the movement of the cry- 

 stals ceases, deserves a closer consideration. 



The behaviour of citric acid enables us to get a better insight into 

 this phaenomenon. The movements of these crystals (which are 

 split into the finest possible plates, in order that they may float upon 

 the water) leave behind them upon the surface of the water, visible 

 traces of dissolved citric acid. After touching the water, the move- 

 ment of the crystal ceases, but its solution continues, but with this 

 difference, that the visible trace of the solution no longer floats upon 

 the surface, but sinks perpendicularly to the bottom. The cause of 

 the cessation of movement must be sought for in some body, which 

 has diffused itself over the surface of the water after contact. This 

 body was ascertained to be an extremely thin stratum of a fatty 

 substance, which had separated from the finger in touching the water 

 and rapidly diffused over the surface. 



The diffusion of this substance on the surface of the water goes 

 so far, that a glass or metal rod touched by the naked hand, when 

 immersed in the water either immediately brings the crystals in full 

 motion to a standstill, or so far diminishes their movement, that it 

 can only be compared to the nervous convulsions of a dying animal. 



This property of extremely fine diffusion upon the surface of 

 water, appears only to be possessed by the fatty oils and their 

 soaps ; for light wood-tar oil, for example, even when thrown upon 

 the water in drops, did not stop the movements of the crystals. 



For the elucidation of these phsenomena, the floating lines of the 

 dissolved citric acid furnish a visible explanation ; notwithstanding 

 their greater specific gravity, they swim upon the surface of the 

 water. But if this surface be covered by another body, which by its 

 greater attraction occupies the surface as such, the solution passes 

 over into the lower mass of water, and being specifically heavier, 

 falls to the bottom. In the first case the surface alone was the 

 solvent. The displacement of the dissolved portion by the constant 

 closing in of the solvent, caused the movement of the floating cry- 

 stals. 



In the second case the surface of the water was occupied by the 



