NATURE OF LIFE AND LIVING MATERIAL 



21 



substances that are wetted by water, that is, they take up or absorb 

 water in a manner similar to the adsorption of water by gelatin. 

 It is quite probable that there is a great variety of suspended sub- 

 stances, some of which may be considered as clumps or aggregates 

 of molecules, in other words, colloidal particles, but many of which, 

 particularly the proteins, are large molecules that because of their 

 large size exhibit the characters of molecular aggregates. One may 

 with some accuracy think of these bodies or large molecules as 

 being surrounded by an envelope of water molecules, the thickness 

 of the envelope varying with the constant changes that are char- 

 acteristic of living protoplasm. This picture gives us a concept, 

 however inaccurate, of the ground substance of protoplasm which 

 under the microscope appears to be structureless. Visible bodies 

 are fibres, granules, vacuoles, rods, and films. 



In order to comprehend the immensity of the problem of under- 

 standing the physico-chemical nature of the processes in protoplasm 

 that result in the phenomena known as irritability, growth, and 

 so on, it is necessary to examine the nature of the behavior of col- 

 loidal systems in general, fundamental properties that are common 

 to non-living as well as living systems. 



Surface Phenomena. Each particle of a colloidal suspension 

 exposes a surface to the suspending medium and the sum of the 

 surfaces in a colloid is a very great area. It is considered that the 

 peculiar properties of colloids that distinguish them from molecular 

 dispersions or true solutions are largely due to the properties of the 

 boundaries, or, surfaces, between the suspended and the suspend- 

 ing media. Since protoplasm is a colloid, it becomes necessary to 

 consider the chief facts concerning surface phenomena, particularly 

 as they are related to colloid behavior. 



Surfaces in Protoplasm. But how much surface is to be 

 found in the protoplasmic colloidal system? It must be understood 

 that when the term surface is used it must be applied to all the 

 boundaries between suspended substances and the external phase, 



