Molecular Motions in Living Bodies. 269 



MOLECULAR MOTIONS IN LIVING BODIES. 



BY HENRY J. SLACK, F.G.S., 

 Member of the Microscopical Society. 



Before suggesting inquiry into the part which molecular mo- 

 tions perform in the growth and decay of living organisms, 

 I shall endeavour to make the subject more generally interesting 

 by a few preliminary observations, which may assist those to 

 whom it is entirely new. 



In order to know what molecular movements are, a small 

 drop of water should be placed on a glass slide, just touched 

 with a fine camel-hair brush whose point has been dipped in 

 gamboge, then covered with a thin glass, and viewed with a 

 g- objective and second eye-piece, or with a higher power, 

 if one is at hand. The scene disclosed to the eye is singularly 

 striking when first observed, and may be frequently seen 

 without losing the interest it originally inspires. Thousands 

 of little round particles are perceived to keep up an active 

 fidgetty motion, sometimes approaching, sometimes receding, 

 rolling, quivering, shaking, and comporting themselves not 

 unlike a swarm of live creatures suddenly frightened and not 

 at all clear what they are about. If the water does not evapo- 

 rate, the spectacle may be watched for hours, until, at length, 

 it usually happens that the particles adhere to the glass, and 

 quiet is restored. 



The French call these movements " Brownian," after their 

 discoverer, the famous English botanist, and they may be pro- 

 duced with any material not soluble in water, provided the 

 size of the particles is proportioned to their own specific gravity 

 and to that of the fluid. What is required is, that the particles 

 shall be freely suspended in the liquid, and be of minute di- 

 mensions. Substances of nearly the same specific gravity as 

 water will have little tendency to rise or fall, and that tendency 

 is easily controlled, for a time, by reducing them to a moderate 

 degree of fineness. The particles of the water cohere with a 

 certain force, so that a greater force is necessary to make any 

 substance move either upwards or downwards in that fluid. 

 It is more easy to move through a light fluid than a dense one. 

 Fresh water, for example, opposes less resistance than salt. 

 Every bather has noticed the difference between trying to touch 

 the bottom in a river and in tho sea, while Dead Sea water is so 

 heavy as to make swimming an easy task for an animal not 

 specifically heavier than a man. In like manner limpid fluids 

 oppose less resistance than sticky ones ; and an insect that can 

 move easily through water, is sadly impeded when immersed in 

 glue. 



