82 Prof. Briicke on Gravitation and 



ments, and this can only mean that force is generated. Both 

 results are inconsistent with the law of the Conservation of Force. 

 If we imagine a particle isolated and without gravitating force, 

 and afterwards introduce another, an attractive force is supposed 

 to be set up on both sides, and thus again, according to the 

 usual conception, we should have a creation of force. 



Further, if we conceive the particle B to be separated from 

 the particle A for an infinite distance, the attractive force is 

 thereby infinitely diminished ; it is, in fact, as if B in resj^ect to 

 A were actually annihilated. The same reasoning may be ap- 

 plied to several particles. When a body approaches another by 

 gravitating motion, in virtue of the vis inertia' a quantity of me- 

 chanical force is accumulated, and still the vis attractionis has 

 not diminished but augmented. If, however, by the application 

 of an external force the body be removed in the opposite direc- 

 tion, there is no force stored up by the act, but, on the contrary, 

 the ris attractionis is, in consequence of the greater distance, 

 diminished. Hence Faraday considers that our present notions 

 as to the cause of gravity cannot be in harmony with the law of 

 the conservation of force, as long as it is not shown whence the 

 force generated is derived, and whither the force lost has disap- 

 peared. He does not doubt the general applicability of the 

 principle of conservation, but he believes that in our definition of 

 gravity wc only describe one exercise of that power, and that this 

 gives an incomplete idea of the nature of the force as a whole. 



It is a long time since such a far-reaching physical question 

 has been touched upon wholly without the aid of mathematical 

 apparatus — without the assistance of those wonder-working sym- 

 bols whose brief rhetoric speaks more convincingly to the mind 

 than the tongue of Cicero or Demosthenes. When, however, 

 the first natural philosopher in the world — when Faraday opens 

 the discussion thus, it is assuredly a sign that the time has come 

 when the question may be so treated. 



If I ventui'c to take part in this discussion, it is not because 

 I feel myself competent to meet that great man upon his own 

 field, but because the subject to which I devote myself, that is, 

 physiology, causes me perhaps to pay attention to things which 

 may have been further from the course of thought of the English 

 philosopher. Habitually compelled to direct my eye upon the 

 inner man, I may i)crhaps be able to make some remarks on the 

 connexion between natural things and our way of representing 

 them to our minds, which will cause the relation between the law 

 of gravitation and that of the conservation of force to appear in 

 a different light. 



What is it in physics that we denote by the name Force ? Man 

 stands before the external world as a camera obscura, on the screen 

 of which Nature perpetually casts her images ; we cannot under- 



