1891] 1(>^ [Bache. 



visible effect thereby produced on the movements of the particles. 

 If the movements had been produced by electric currents, then so 

 strong a current as I often passed through a drop of water ought to 

 have left no manifestation of movement possible from the neces- 

 sarily weak, if actually existing, currents supposed to be actuating 

 the particles. The molecular movement, so-called, is, as described 

 by Herr Wiener, a zigzag one, but that term does not exactly con- 

 vey the peculiarity of the motion. It is a combination of a jerky, 

 wobbling movement, performed within determinate bounds, entirely 

 irrespective of the sweep of currents in the liquid, or even of the 

 effects in some cases of terrestrial gravity, and in no case seemingly 

 affected by the influence of local gravitation of particle to particle. 

 Taking the vermilion of the sulphide of mercury, as finely divided 

 as it can be made, and turning the microscope at even a slight 

 angle from the vertical, the effect of terrestrial gravity on the par- 

 ticles becomes at once apparent, but taking the carmine, reputedly 

 made of cochineal, the particles are not affected in the slightest 

 degree by terrestrial gravity. Of course it is hardly necessary to 

 say that any solution should be weak, in order to allow the sub- 

 stances under examination to receive the finest division of which 

 each is susceptible as dissolved. T)ie specific gravity of sulphide 

 of mercury is not only much greater than that of cochineal, but 

 additionally it is not susceptible of nearly so minute division as 

 cochineal is. Of all substances that I experimented with, cochineal 

 seemed to be that which is capable of the finest division, and at the 

 same time of the most brilliant illumination. Gamboge, which 

 appears to be the substance of predilection among many persons to 

 experiment with for the brownian movements, offers nothing com- 

 parable to the brilliancy and the fineness of particles of carmine 

 derived from cochineal. With a weak aqueous solution of carmine 

 one may see by daylight, on a background of faint blue, and by 

 ordinary artificial light, on a golden one, thousands of tiny parti- 

 cles, bright as sparks of ruby, shimmering and performing their 

 independent evolutions over the field of view. 



Just as one sees a boat managed by an unskillful helmsman pursue 

 its erratic way in going about, being taken aback, or heeled over by 

 a flaw of wind, without for a moment attributing its movements to 

 currents or any other cause but the true one, so the constant observer 

 of the brownian movements knows full well that the particles them- 

 selves are moving, not being moved by currents or by gravitation 



