PHYSICAL SCIENCE. 15 



of its nature and essence we are utterly ignorant. Science 

 has dealt with its effects only, without really approaching 

 a step nearer to the cause, than when Newton declared that 

 he must leave to the consideration of his readers the question 

 whether the agent producing gravity was material or not. 

 Hypotheses have grown up such as that of gravific atoms 

 permeating all space, of Le Sage; or the residual force 

 theory of Mosotti, connecting gravity with molecular attrac- 

 tions but none which satisfy fully the exigencies of the 

 case. The research is even made more difficult by the sim- 

 plicity and invariability of the power in question. It evolves 

 in certain cases, and constantly controls or modifies, the other 

 forces acting on matter ; but has no such relations to them 

 as they have to one another ; no reciprocal production or 

 mutual convertibility; nor the duality of action belong- 

 ing peculiarly to the electrical and magnetic forces ; nor 

 lines of propagation and polarisation, such as we recognise in 

 light and heat ; nor those molecular changes manifested in 

 acts of chemical affinity. Whether any or if any, through 

 what new avenues closer approach may hereafter be made 

 to the solution of this great problem of gravity, bringing it 

 into closer connection with the other great forces of nature, 

 we cannot here enquire. But in speaking of the forces which 

 act upon matter, it was impossible to omit this the most 

 universal of -all innate and incorporate we might almost 

 say, in matter itself. 



Nor can we rightly avoid in this place some allusion to the 

 equally abstruse subject (though rendered so by very different 

 causes) of the mutual relations of the physical and vital forces 

 a topic handled with great ability by Dr. Carpenter, in a 

 paper in the ' Philosophical Transactions,' a few years ago, 

 and more recently in the systematic works of this physio- 

 logist. Without going into the depths of this question, we 

 may say that the tendency of all recent research has been to 



