''SOME UNRECOGNIZED LAWS OF NATURES 781 



since bodies fall more readily in vacuo than in air. There is an 

 important distinction between kinematic and dynamic reactions. 

 The first, or primary reactions, not only can but always do take 

 place at a distance, and in view of the undoubted fact any discus- 

 sion of the possibility is idle in the extreme. But it is very differ- 

 ent in the case of secondary or dynamic effects. There is, strictly 

 speaking, no reaction between the bodies concerned. They have 

 no attraction for each other. They merely happen to get in each 

 other's way when one or both are undergoing some kinematic 

 reaction. A falling body may meet a second body thrown up- 

 ward, and the path of each will be notably altered, but the reac- 

 tion will be purely dynamic, and as such absolutely dependent on 

 contact. These secondary reactions happen to be the ones with 

 which we are the more familiar and upon which we have allowed 

 our minds to play the more freely. The old question, " Can a 

 body act where it is not ? " is only applicable to dynamic reac- 

 tions, but is not at all pertinent in the discussion of kinematic 

 reactions. In the first case, no one doubts the inability of a body 

 to push or pull another without actual contact. In the second 

 case, the facts leave no room for discussion. 



The concluding book, on gravitation, is naturally the apiece de 

 resistance of the whole, and seems to us of high interest and im- 

 portance. The other books lead up to it very cleverly. Newton 

 himself, as we have seen, had no theory of gravitation, or at least 

 was extremely careful not to publish it if he ever had one. 

 Later disciples, however, have been less cautious, and the ISTewto- 

 nian view has been erected into something of a theory. This 

 assumes that all matter attracts all matter, and this quite inde- 

 pendently of its state of excitation. The intensity of the attrac- 

 tion has been formulated in the well-known law that gravity is 

 proportional to the mass and inversely proportional to the square 

 of the distance. But mass, as Newton used the term, is synony- 

 mous with weight, and weight is simply the measure of gravity, 

 so that this fine-sounding phrase amounts only to this, that gravity 

 is proportional to gravity, a statement which can not be said to 

 materially help on the cause of truth. The Newtonian view as- 

 sumes the constancy of mass or weight, but does so without the 

 least experimental verification, and indeed in the teeth of much 

 contrary evidence. It is true that modern physicists distinguish 

 between mass and weight, making the former term stand for 

 amount of matter (whatever that may mean) and the latter for 

 the measure of gravity. But in this sense mass is a purely meta- 

 physical quantity. 



Now, there are two classic experiments for determining the 

 density of the earth that seem at first sight to establish the cur- 

 rent view that all matter attracts all matter. These are the 



