28 THE DATA OF BIOLOGY. 



Colloids take up by a power called " capillary affinity/' a 

 large quantity of water: undergoing at the same time great 

 increase of bulk with change of form. Conversely, with 

 like readiness, they give up this water by evaporation; 

 resuming, partially or completely, their original states. 

 Whether resulting from capillarity, or from the relatively 

 great diffusibility of water, or from both, these changes are 

 to be here noted as showing another mode in which the 

 arrangements of parts in organic bodies are affected by me- 

 chanical actions. 



In what is termed osmose, we have a further mode of an 

 allied kind. When on opposite sides of a permeable septum, 

 and especially a septum of colloidal substance, are placed 

 miscible solutions of different densities, a double transfer 

 takes place: a large quantity of the less dense solution finds 

 its way through the septum into the more dense solution; 

 and a small quantity of the more dense finds its way into 

 the less dense — one result being a considerable increase in 

 the bulk of the more dense at the expense of the less dense. 

 This process, which appears to depend on several conditions, 

 is not yet fully understood. But be the explanation what it 

 may, the process is one that tends continually to work altera- 

 tions in organic bodies. Through the surfaces of plants and 

 animals, transfers of this kind are ever taking place. Many 

 of the conspicuous changes of form undergone by organic 

 germs, are due mainly to the permeation of their limiting 

 membranes by the surrounding liquids. 



It should be added that besides the direct alterations which 

 the imbibition and transmission of water and watery solutions 

 b} colloids produce in organic matter, they produce indirect 

 alterations. Being instrumental in conveying into the tissues 

 the agents of chemical change, and conveying out of them 

 the products of chemical change, they aid in carrying on 

 other re-distributions. 



§ 12. As elsewhere shown {First Principles, § 100) heat, 



