16 INTRODUCTORY [ch. 



or movement, or the movements that change of form impHes, then 

 Force is the appropriate term for our conception of the causes by 

 which these forms and changes of form are brought about. When 

 we use the term force, we use it, as the physicist always does, for 

 the sake of brevity, using a symbol for the magnitude and direction 

 of an action in reference to the symbol or diagram of a material 

 thing. It is a term as subjective and symbolic as form itself, and 

 SO' is used appropriately in connection therewith. 



The form, then, of any portion of matter, whether it be living 

 or dead, and the changes of form which are apparent in its movements 

 and in its growth, may in all cases alike be described as due to 

 the action of force. In short, the form of an object is a "diagram 

 of forces," in this sense, at least, that from it we can judge of or 

 deduce the forces that are acting or have acted upon it: in this 

 strict and particular sense, it is a diagram — in the case of a sohd, 

 of the forces which have been impressed upon it when its conformation 

 was produced, together with those which enable it to retain its 

 conformation; in the case of a Hquid (or of a gas) of the forces which 

 are for the moment acting on it to restrain or balance its own 

 inherent mobility. In an organism, great or small, it is not merely 

 the nature of the motions of the hving substance which we must 

 interpret in terms of force (according to kinetics), but also the 

 conformation of the organism itself, whose permanence or equilibrium 

 is explained by the interaction or balance of forces, as described in 

 statics. 



If we look at the hving cell of an Amoeba or a Spirogyra, we 

 see a something which exhibits certain active movements, and a 

 certain fluctuating, or more or less lasting, form; and its form at 

 a given moment, just like its motions, is to be investigated by the 

 help of physical methods, and explained by the invocation of the 

 mathematical conception of force. 



Now the state, including the shape or form, of a portion of matter 

 is the resultant of a number of forces, which represent or symbolise 

 the manifestations of various kinds of energy; and it is obvious, 

 accordingly, that a great part of physical science must be under- 

 stood or taken for granted as the necessary preliminary to the 

 discussion on which we are engaged. But we may at least try to 

 indicate, very briefly, the nature of the principal forces and the 



