270 Molecular Motions in Living Bodies. 



When any insoluble body is pressed under water, it dis- 

 places its own bulk of that fluid. If it is lighter than that bulk, 

 it is forced up, and floats. If heavier, it is forced down by 

 gravitation, and sinks. But although the specific gravity of a 

 substance is greater than that of water, it will still float, pro- 

 vided its surface is extended, so that the resistance of the 

 water, arising from the cohesion of its particles, is made equal, 

 or more than equal to, the weight of the substance, or force, 

 with which it gravitates. Thus a film of gold leaf will float, 

 while the same weight of gold in a pellet falls fast. From 

 these facts it results that in order to cause a heavy metal like 

 gold or platina to be suspended in water, with little tendency 

 to fall, its particles must be reduced to such a degree of 

 fineness that their weight is nearly counterbalanced by the 

 resistance which the fluid offers to the passage of their bulk. 

 This can be accomplished more easily than might be expected, 

 because the weight of round bodies diminishes much faster 

 than their size. The rule is, that the contents of spheres are as 

 the cubes of their diameter, so that if a ball three inches in 

 diameter weighed 27, another ball one inch in diameter would 

 only weigh 1, Thus a moderate reduction in the size of a 

 round particle makes a great deal of difference in its weight. 



When a minute particle is freely suspended in a highly 

 mobile fluid like water, the slightest force of any kind will 

 disturb its equilibrium, and impart some motion ; but exactly 

 what force causes the molecular movements does not appear to 

 have been ascertained. Dr. Carpenter gives an interesting 

 summary of what is known, in his work on the Microscope, 

 from which we will make a quotation. He says : — - 



" Nothing is better adapted to show it (the molecular 

 motion) than a minute portion of gamboge, indigo, or carmine, 

 rubbed up with water, for the particles of these substances that 

 are not dissolved, but only suspended, are of sufficiently large 

 size to be easily distinguished with a magnifying power of 250 

 diameters, and are seen in perpetual locomotion. Their move- 

 ment is chiefly of an oscillatory kind, but they also rotate back- 

 wards and forwards upon their axis, and [hey gradually change 

 their places in tin' field of view. It may be observed that the 

 movement of the smallest particles is the most energetic, and 

 that the largest ore quite motionless, while those of inter- 

 mediate size move but with comparative inertness. The move- 

 ment 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 evapora- 

 tion ; and it has been known to continue for many years in a 

 small quantity of fluid enclosed between two glasses in an air- 



