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LIIL On the Importance of Experiments in relation to the Me- 

 chanical Theory of Gravitation. By S. Tolver Preston*. 



THE assumption of " action at a distance " has notoriously 

 tended to check experimental inquiry in relation to the 

 mechanical cause of gravitation, as it has hindered investiga- 

 tion in other branches of physics. At one time, for instance, 

 on the basis of this theory of "action at a distance," electric 

 action was supposed to propagate itself in some mysterious 

 way across space without the intervention of matter ; so that 

 all substances were assumed to play a perfectly neutral part 

 in the passage of the electric action — until Faraday, rising 

 above the prejudices of his time, put this question under the 

 test of experiment, and by his discovery of a " specific induc- 

 tive capacity/' demonstrated the supposition thus formed to 

 be false. Yet so great was the strength of preconceived 

 opinion, that notoriously Faraday's discovery was not fully 

 accepted until years afterwards, when its practical application 

 to telegraph-cables forced it on the attention of electricians. 



So now in the present day, in spite of the continual demo- 

 lition of spiritualistic views (i. e. views of action without the 

 intervention of matter) by advancing science, the modus ope- 

 randi of gravitation remains the one outstanding subject for 

 doubt ; and it appears to be assumed by many (much in ana- 

 logy with the former question of "specific inductive capacity ") 

 that experiments with the view to decide whether any minute 

 specific differences might exist in the effects of gravity, de- 

 pendent on structure, form, &c. of substances, would be out of 

 place — gravity being rather considered as something not phy- 

 sical at all (although admitted, as it were, by courtesy into 

 " physical " science). 



No doubt it may be agreed at the outset here that such 

 specific differences in gravific effects (if they exist) must be 

 very small, or they would have been accidentally detected. 

 But the history of science shows that phenomena are often not 

 detected by accident, but have to be carefully searched for. 

 Where would the discovery of diamagnetism (for example) 

 have been without a careful investigation ? Here it w T as gene- 

 rally assumed beforehand that, with the exception of certain 

 isolated magnetic substances (iron &c), no specific differences 

 would exist in the behaviour of bodies in general to magnetism 

 (which was, in the same way, supposed to be an occult kind 

 of " action at a distance "), until Faraday, again resorting to 

 rigorous experiment, broke down once more this belief, and 

 that only after the most careful and laborious investigations. 



* Communicated by the Author. 



