MATTER AND ENERGY 81 



complex chemical constitution. They all 

 contain about 16 per cent, of nitrogen, along 

 with carbon (more than half their weight), 

 hydrogen, oxygen, and usually a small amount 

 of sulphur or phosphorus, or both. Proteins, 

 a typical example of which we find in the 

 albumen in white of egg, are essential in 

 protoplasm, and they are more intimately 

 associated with the phenomena of life than 

 any of the other proximate principles, in the 

 sense that w r e never find vital phenomena 

 without them, and that vital phenomena are 

 never manifested by carbo-hydrates, fats, saline 

 matter, or water, either alone or in combination. 

 Proteins are usually colloidal or glue-like, and 

 are non-diffusible through animal membranes. 

 A colloid does not form a true solution, but in a 

 fluid it forms a kind of emulsion consisting of 

 minute particles or globules suspended in the 

 fluid. (Such an emulsion- colloid is termed 

 a gel, but there are colloids, having much 

 finer particles, and which have different 

 properties. Such are called sols. Proto- 

 plasm, alive, is probably of the nature of a 

 sol.) 



