154 THE MICROSCOPE AND IT8 REVELATIONS. 



of fine clay (such as the prepared kaolin used by Photographers) shaken- 

 np with water, is recommended by Prof. Jevons, 1 who has recently 

 studied this subject, as showing the movement (which he designates 

 pedesis) extremely well. But none of the particles he has examined are 

 so active as those of pumice-stone that has been ground-up in an agate 

 mortar; for these are seen under the microscope to leap and swarm with 

 ,an incessant quivering movement, so rapid that it is impossible to follow 

 the course of a particle which probably changes its direction of motion 

 15 or 20 times in a second. The distance through which a particle moves 

 at any one bound is usually less than l-5000th of an inch. This 

 ' Brownian movement ' (as it is commonly termed) is not due to evapora- 

 tion of the liquid : for it continues, without the least abatement of energy, 

 in a drop of aqueous fluid that is completely surrounded by oil, and is 

 therefore cut off from all possibility of evaporation; and it has been known 

 to continue for many years in a small quantity of fluid inclosed between 

 two glasses in an air-tight case. And, for the same reason, it can scarcely 

 be connected with chemical change. But the observations of Prof. 

 Jevons (loc. cit.) show that it is greatly affected by the admixture of 

 various substances with water; being, for example, increased by a small 

 .admixture of gum, while it is checked by an extremely minute admixture 

 of sulphuric acid or of various saline compounds, these (as Prof. J. points 

 out) being all such as increase the conducting power of water for Elec- 

 tricity. The rate of subsidence of finely-divided clays or other particles 

 suspended in water, thus greatly depends upon the activity of their 

 ' Brownian movement;' for, when this is brought to a stand, the particles 

 aggregate and sink, so that the liquid clears itself . In any case in which 

 the motions of very minute particles, of whatever kind, are in question, 

 it is necessary to make allowance for this ' molecular ' movement; and the 

 young Microscopist will therefore do well to familiarize himself with its 

 ordinary characters, by the careful observation of it in such cases as those 

 just named, and in any others in which he may meet with it. 2 



156. Diffraction. The course of Light-rays is altered not only by 

 refraction when they pass from one transparent medium into another, 

 and by reflection when they fall on polished surfaces which they do not 

 enter, but also by inflection at the edges of objects by which they pass; 

 and as the differently colored rays which altogether make up white light 

 are affected by such inflection in different degrees, they are separated by 

 it (as by refractive ' dispersion ') into colored bands; the phenomenon be- 

 ing altogether known as diffraction. This may be made evident by caus- 

 ing abeam of sunlight to enter a darkened room through a very narrow 

 slit, and to fall on a white screen; for the narrow line of white light will 

 show a border of colored fringes, which become wider as the slit is nar- 

 rowed; and if these fringes be viewed through a piece of colored glass, 

 which allows only rays of its own color to pass, they will appear as a suc- 

 cession of bands alternately bright and dark. This alternation is pro- 

 duced by the interference of the Light-waves; just as the alternations of 

 sound and comparative silence termed ' beats/ which are heard when two 

 .slightly different tones are being sounded together, are due to the inter- 



1 "Quarterly Journal of Science," N. S., Vol. viii. (1878), p. 172. 



'See also the Rev. J. Delsaulx "On the Thermo-Dynamic Origin of the 

 Brownian Motions" in "Monthly Journ. of Microsc. Sci.," Vol. xviii. (1877), p. 1; 

 and Dr. W. M. Ord "On some Causes of Brownian Movements" in "Journ. of 

 Hoy. Microsc. Soc.," Vol. ii. (1879), p. 656. 



