12 GENERAL HISTOLOGY. 



2. Diffusion relates to the tendency of fluids or gases to 

 mix intimately even contrary to specific gravity. This tend- 

 ency varies in different substances. Thus a solution of com- 

 mon salt, which is a crystalloid, has a diffusive power of 

 twenty times as great as a solution of albumen (a colloid) of 

 equal strength. 



3. Osmose refers to the passage, or separation, of fluids or 

 gases by a membranous or porous diaphragm. Thus in Du- 

 trochet's experiment a membrane or bladder was tied over the 

 large end of a funnel containing colored alcohol. This was 

 inverted in a vessel of water, which fluid passes through the 

 membrane, causing the alcohol to overflow. The process of 

 separation of materials by a membrane, etc., is called dialysis, and 

 the passage through the membrane, or osmose, is called en- 

 dosmose when it passes inwards, and when outwards exos- 

 mose. For dialysis the membrane must be moist, hence 

 the advantage of wetting the skin in fevers. Alkaline solu- 

 tions generally exhibit endosmose, and acids exosmose. 

 Since colloid material does not readily permeate a porous dia- 

 phragm, while crystalline solutions pass readily, crystalline 

 poisons may be separated, for examination, from colloid animal 

 fluids, as in the contents of the stomach. The colloid state is 

 not peculiar to organic matter, as seen in hydrated silicic acid, 

 soluble alumina, etc. As colloid substances are as permeable to 

 fluids as membrane, the colloid state of bioplasm permits the 

 flow of currents, as in ordinary dialysis, modified, perhaps, by 

 vitality. 



4. All physical forces, as light, heat, electricity and gravi- 

 tation, act in and upon bioplasm as well as non-living matter, 

 yet vital influence so modifies these forces as to show that it is 

 not identical with them. Gravity tends to bring all things 

 towards the center of the earth, yet a living tree grows up- 

 wards in opposition to gravity. Different bioplasts differ 

 greatly as to endurance of heat. Some are deprived of life 

 by a freezing temperature, and many by a heat much below 

 that of boiling water. Yet trout's eggs develop well in ice- 

 water, and die at a moderate temperature, while certain con- 

 fervse have their habitat in the boiling water of sulphurous 



