320 THE GRAMMAR OF SCIENCE 



(Appendix, Note III?) ; we must ask, when we proceed to 

 extend it beyond the range where it has been found to 

 describe experience, whether it still suffices to simplify 

 our conceptions, or leaves undescribed certain recognised 

 phases of perception. Newton's law appears perfectly 

 sufficient, and may therefore be said to be verified, when 

 we are dealing with particles of gross " matter." The 

 mutual accelerations, for example, of two gravitating 

 particles seem to be uninfluenced by the presence of a 

 third particle ; there is nothing, to take a still more con- 

 crete example, yet observed which would compel us to 

 conceive that the mutual accelerations, by which we 

 describe the mutual dance of sun and earth, are in the 

 least influenced by the presence of the moon. Yet when 

 we come to extend this law of Newton's, invaluable as 

 it is for dealing with particles of gross " matter," to the 

 mutual action of molecules, atoms, and ether-elements, 

 there appears to be considerable reason for doubting its 

 accuracy. 



We can conceive atomic structures — for example, the 

 ether-squirt — for which modified action is essentially true. 

 There are phenomena of cohesion which can hardly be 

 described without supposing the action of two molecules 

 A and B to be modified by the presence of a third mole- 

 cule C.^ There are chemical facts which suggest that the 

 introduction of a third atom C may even reverse the sense 

 of the mutual accelerations of two atoms A and B. Nay, 

 those who, in order to describe the radiation of light, treat 

 the ether as an elastic jelly (p. 263), will find that it is 

 very difficult to conceptualise its elastic structure, without 

 asserting that the hypothesis of modified action is true of 

 the ether-elements. The parallelogram of forces, then, as 

 a synthesis of motion must be considered as applying in 

 the first place to particles of gross " matter " ; its exten- 

 sion to other corpuscles can only be made cautiously and 



1 A fuller discussion of "aspect" and "modified action" by the present 

 writer will be found in Todhunter and Pearson's History of Elasticity, vol. i. 

 arts. 921-31, 1527, and vol. ii. arts. 276, 304-6. See also the American 

 Journal of Mathematics, vol. xiii. pp. 321-2, 345, 353, 361. 



