MOLECULAK MOVEMENT. 169 



spot of greatest luminosity will be found by causing the object 

 glass to approach ivitkin the proper focus. — Now in the 'protoplasm ' 

 of the cells of the lower Plants, and in the ' sarcode' of the lower 

 Animals, oil-particles and vacuoles (or void spaces) are often inter- 

 spersed ; and these at first sight present so very striking a resem- 

 blance, that the inexperienced observer may well be pardoned 

 for mistaking the ' vacuoles ' for larger globules of a material more 

 refractive than the gelatinous substance around them. But the 

 difference in the effects of alterations of focus on the two sets of 

 appearances at once serves to make evident the difference of their 

 causes ; and this, moreover, is made obvious by the effect of oblique 

 light, which will cause the strongest shadow to exhibit itself on 

 opposite sides in the two cases respectively. 



130. Among the sources of fallacy by which the young Micro- 

 scopist is liable to be misled, one of the most curious is the 

 Molecular Movement which is exhibited by the particles of nearly 

 all bodies that are sufficiently finely divided, when suspended in 

 water or other fluids. This movement was first observed in the fine 

 granular particles which exist in great abundance in the contents of 

 the Pollen-grains of plants (sometimes termed the fovilla), and 

 which are set free by crushing them ; and it was imagined that 

 they indicated the possession of some special vital endowment by 

 these particles, analogous to that of the Spermatozoa of animals. In 

 the year 1827, however, it was announced by Dr. Robert Brown that 

 numerous other substances, Organic and Inorganic, when reduced 

 to a state of equally minute division, exhibit a like movement, so 

 that it cannot be regarded as indicative of any endowment peculiar 

 to the fovilla -granules ; and subsequent researches have shown that 

 there is no known exception to the rule, that such motion takes 

 place in the particles of all substances, though some require to be 

 more finely divided than others before they will exhibit it. Nothing 

 is better adapted to show it than a minute portion of Gamboge, 

 Indigo, or Carmine, rubbed up with water ; for the particles of 

 these substances which are not dissolved, but only suspended, are 

 of sufficiently large size to be easily distinguished with a magnify- 

 ing power of 250 diameters, and are seen to be in perpetual 

 locomotion. Their movement is chiefly of an oscillatory kind ; but 

 they also rotate backwards and forwards upon their axes, and they 

 gradually change their places in the field of view. It may be 

 observed that the movement of the smallest particles is the most 

 energetic, and that the largest are quite motionless, whilst those 

 of intermediate size move but with comparative inertness. The 

 movement is not due (as some have imagined) to evaporation 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 enclosed between two glasses in an air-tight case. It is, 



