26 THE DATA OF BIOLOGY. 



as well as other matter that assumes the same state of mole- 

 cular aggregation. 



Colloids take up by a power that has been 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 more or less 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 arrangement of parts in organic bodies, is affected by 

 mechanical forces. 



In what is called osmose, we have a further mode of 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 quan- 

 tity 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 un- 

 derstood. But be the explanation what it may, the process 

 is one that tends continually to work alterations in organic 

 bodies. Through the surfaces of plants and animals, transfers 

 of this kind are ever taking place. Very many of the con- 

 spicuous 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 

 by 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. 



