736 The Outline of Science 



substances can exist as both, it is better to speak of the colloidal 

 and crystalloid conditions of matter. Moreover, the colloidal 

 condition may be fluid, and then it is called a "sol"; or it may be 

 gelatinous (even solid) , and then it is called a "gel." In any case, 

 "matter in the colloidal state is in the form of ultra-microscopic 

 particles of solid, or droplets of fluid, in suspension or in other 

 manner of dispersion, in another solid, liquid, or gas." The study 

 of the properties of the colloidal state has proved of great im- 

 portance in chemistry, and it is of fundamental interest to the 

 physiologist, for living matter is in a colloidal state, Professor 

 Bayliss writes: 



Protoplasm in the living state has the properties of a liquid 

 system, containing, however, particles of solids and droplets 



of immiscible liquids in a freely movable state. 

 * 



Many vital phenomena are bound up with the colloidal properties 



of the physical basis of life. 



We have said that many of the rare elements, though very 

 important to us, lurk quietly in remote corners. 



Rare Earths and their Uses 



Iron is common, but gold is comparatively rare. To get two 

 ounces of the metal gallium a great chemist had to work over 

 nearly 600 pounds of crude material, but aluminium is easily pro- 

 cured. Why are half of the known elements so rare that few 

 people know their names? As Professor R. K. Duncan writes, 



Who, for example, has ever heard of, much less worked with, 

 lanthanum, europium, erbium, neodynium, gadolinium, thu- 

 lium, praseodymium, terbium, ytterbium, samarium, hol- 

 mium, tantalum? Yet these forbidding names represent 

 certain elements of matter just as do others, such as sulphur, 

 phosphorus, or lead ; elements, too, unique in their properties 

 and filled with all kinds of potential usefulness to man. 



We cannot answer this plain question : Why are many of the 

 elements so rare? It may be that they are rare because they are 



