CHAP. II.] MOLECULAR MOTION. 59 



in the particles of various organic and inorganic substances in a 

 state of extremely minute subdivision. When these particles were 

 suspended in water, they exhibited, under the microscope, motions, 

 which consisted in more or less rapid oscillations and rotations of 

 the particles themselves. He found them in the pollen of plants, 

 in many mineral and metallic substances, in various animal matters, 

 reduced to a subtle powder, consisting of particles that ranged in 

 diameter between the y^^- g- and -^-Q^-^Q of an inch. The move- 

 ments are clearly not peculiar to living or organic parts, for they 

 occur in inanimate ones : they never occur excepting when the par- 

 ticles are suspended in water, or some liquid ; and they are attri- 

 butable to currents produced in the fluid by evaporation at its 

 surface or edges, for they may be arrested by covering the fluid 

 with oil, or using other means to prevent such evaporation. They 

 are not, therefore, inherent in the particles themselves, which only 

 obey the impulse communicated to them by the currents created in 

 the fluid which holds them in suspension. 



Certain particles, naturally very minute, which are met with in 

 the body, exhibit motions when examined under the microscope, 

 floating in fluid. These motions are entirely due to the same cause 

 as would excite them in inorganic particles, namely, to currents in 

 the fluid created by its evaporation. The minute granules, or par- 

 ticles of the chyle, have been found to exhibit molecular motion ; 

 and it has been ingeniously supposed, that " these motions indicate 

 the first obvious impress of vitality which the new material has 

 received from its association with living matter." But, before this 

 supposition can be admitted, we require evidence to shew that these 

 motions are inherent, and do not result from currents. The minute 

 rod-like bodies, which form the outer coat of the retina, or Jacob's 

 membrane, also sometimes exhibit a molecular motion, when sepa- 

 rated and examined in water, and there seems no reason to doubt 

 that this is due to the evaporation of the fluid in which they float. 



2. Organic Molecular Motion. Some of the motions, which take 

 place within the living body, may be compared to those described 

 by Mr. Brown, inasmuch as they are generally, if not always, due 

 to an extraneous force acting upon the particles moved ; but they 

 differ in this respect, viz. that the producing force is developed by 

 the processes inseparable from life. Such motions are not in gene- 

 ral visible, yet some have been seen, which clearly indicate that 

 forces are developed, during life, capable of producing them. The 

 movements of particles within cells afford an example : such 

 motions are cither of a uniform rhythmical kind, or they are 



