1894.] -»-t>5 [Bache. 



think, be satisfactorily shown by means of a very simple experi- 

 ment." [Dr. Brown here alludes to what he details at considerable 

 length, as to the trituration together of oil and water, so as to 

 secure, in one case, by a large proportion of water to oil, lacunae of 

 water of various sizes, filled with particles, protected from evapora- 

 tion by the oil, and in the converse case, by a small proportion of 

 oil to water, to secure minute oil-drops on the surface of the water, 

 some of which drops were not larger than the solid particles in the 

 water.] 



These passages from the writings on this subject of so skillful, 

 careful, and conscientious an observer as Dr. Brown was, excite 

 regret that he had not pursued the subject further. He would have 

 found, among other things, a much simpler and more lasting 

 method of excluding air from the drop under observation, than 

 that which he adopted in forming lacunae of aqueous fluid pro- 

 tected from evaporation by immersion in oil. He would in all 

 probability have discovered the real cause of the motion of or- 

 ganic and inorganic particles in aqueous fluids. But he dropped 

 the subject, perhaps because he was obliged so to do on account of 

 the pressure of other research, when he had obtained some valuable 

 results, leaving his investigations negative as to conclusions. He 

 doubtless began with the supposition, which seems common at first 

 to all investigators of the subject, that evaporation might represent 

 a shock that would move the particles suspended in water; that 

 vibration from mechanical sources might account for the move- 

 ments; that currents set up in the drop of water by differences of 

 temperature in it or slight differences in the temperature of the air 

 surrounding it, might account for them ; or that mutual attractions, 

 derived from gravitation, and inherent in the relative density of the 

 particles themselves, might do so ; and he found, as every one else 

 will find who experiments in these directions, that the movements 

 go on independently of currents and independently of heat, generat- 

 ing or not generating currents, independently of light, and in the 

 case where the particles represent an extremely fine division of 

 matter, and are at the same time of low specific gravity, even inde- 

 pendently of terrestrial gravitation ; and without relation to their 

 specific gravity, with the force of their mutual gravitation entirely 

 masked. 



Some of these things Dr. Brown implies that he saw in the course 

 of his investigations; others he could hardly have failed incidentally 



