206 Mr. S. T. Preston on some Dtjnamical Conditions 



sions as " attached to a carbon atom at the end of the chain," 

 &c. &c., especially in text-books for students (who generally 

 find it hard to distinguish fact from theory), and unaccom- 

 panied by any warning, or explanation of the real value of the 

 expressions, is, I think, much to be deprecated. It behoves us 

 ever to bear in mind that in science we deal with phenomena 

 made up of an infinite number of infinitely small units, and 

 that therefore any deficiency in careful measurement may 

 lead to the supposition of a causal connexion where no such 

 connexion exists, that even at the best we gain only approxi- 

 mations, and that from the most carefully conducted experi- 

 ments we cannot tell what may happen in extreme cases 

 (Principles of Science)*. 



[To be continued.] 



XXY. On some Dynamical Conditions applicable to Le Sage's 

 Theory of Gravitation. By S. ToLVER PRESTONf. 



1. rjlHE tendency of modern science is undoubtedly to look 

 J- to the existence of physical conditions or processes 

 in those natural phenomena to which the theory of " action at 

 a distance " has been applied. The gravitation theory of Le 

 Sage has therefore of late naturally received a considerable 

 share of attention. Le Sage finds it necessary, as a basis to 

 his theory, to lay down certain conditions, some. of which can- 

 not but be regarded as arbitrary. Thus (as given in the paper 

 bv Sir William Thomson, ^ Philosophical Magazine,' May 

 1873) Le Sage assumes among other conditions: — 



(1) That the direction of the streams of particles producing 

 gravity is such that an equal number of particles are moving 

 in all directions. 



(2) That the streams are all equally dense \ or the total 

 assemblage of matter forming the streams is of the same den- 

 sity in all parts. 



(3) That the mean velocity of the streams is everywhere the 

 same. 



2. These conditions cannot but be considered arbitrary. 

 My object is to call attention to the fact (which, if it has been 

 observed, would certainly appear to be deserving of more 

 attention than it has received) that all these conditions 

 which Le Sage, with the limited knowledge of his day, assumed 

 to be arbitrary, are in reality inevitable deductions following 



* For a fuller discussion of the constitutional formul?e of alcohols and 

 acids &c., see Die modernen Tlieoricn, 2nd edit. pp. 190-202. 

 t Communicated bv the Author. 



