3IO THE GRAMMAR OF SCIENCE 



may be tested by weighing. Another important result 

 also flows from this discussion. If a particle suspended 

 by a string be at rest relative to the earth, then its 

 weight will be equal to the tension in the string. Hence, 

 if the earth-acceleration g at any place be known, we 

 have a means of measuring mass in terms of tension. A 

 further development of this principle forms the basis of 

 important methods of determining the equality of masses 

 by the equality of strains (p. 202) due to equal tensions. 



S 1 1 . — How far does the MecJianism of the Fourtli and 

 Fifth Laws of Motion extend? 



Before we conclude this discussion of mass, there are 

 still several points with regard to it which must be 

 elucidated even in an elementary work like the present. 

 We have first to ask whether our fourth and fifth laws of 

 motion, with the definitions of mass and force involved in 

 them, must be conceived as holding for the whole range 

 of corpuscles from ether-element to particle. The same 

 difficulty, of course, arises with regard to force as arose 

 with regard to acceleration, if we conceive prime-atoms as 

 possibly, and chemical atoms and molecules as almost 

 certainly, extended bodies. There cease to be definite 

 points between which the mutual accelerations, and 

 accordingly the forces, have their directions. We are 

 thrown back on the conception that if these laws are to 

 be applied to atoms and molecules, it must be to the. 

 action and reaction between the elementary parts of those 

 corpuscles and to the masses of the elementary parts that 

 our laws refer. From the action of these elementary 

 parts on each other we must, then, deduce by aid of the 

 above laws the total action between two atoms or two 

 molecules. This will not necessarily be measurable by 

 a single force acting between two definite points. 



Further difficulties, however, arise with regard to our 

 conception of mass. Is the mass of an ether-element 

 of the same character as the mass of an atom, or a mole- 



