EXPERIMENT. 45 



upon a mixture compounded so as to contain a known 

 quantity of that element. The accuracy of the gold assay 

 process greatly depends upon the precaution of assaying 

 alloys of gold of exactly known composition 111 . Gabriel 

 Plattes' works give evidence of much scientific spirit, and 

 when discussing the supposed merits of the divining rod 

 for the discovery of subterranean treasure, he sensibly 

 suggests that the rod should be tried in places where veins 

 of metal are know r n to exist, and, we might add, known not 

 to exist". 



Negative Results of Experiment. 



When we pay proper regard to the imperfection of all 

 measuring instruments and the possible minuteness of 

 effects, we shall see much reason for interpreting with 

 caution the negative results of experiments. We may 

 fail to discover the existence of an expected effect, not 

 because that effect is really non-existent, but because it 

 is of an amount inappreciable to our senses, or confounded 

 with other effects of much greater amount. As in fact 

 there is no limit on a priori grounds to the smallness of a 

 phenomenon, we can never, on the grounds of a single ex- 

 periment, prove the non-existence of a supposed effect. 

 We are always at liberty to assume that a certain amount 

 of effect might have been detected by greater delicacy of 

 measurement. We cannot safely affirm that the moon has 

 no atmosphere at all. We may doubtless show that the 

 atmosphere, if present, is less dense than the air in the 

 so-called vacuum of an air-pump, as did Du Sejour. It is 

 equally impossible to prove that gravity occupies no time 

 in transmission. Laplace indeed ascertained that the 

 velocity of propagation of the influence was at least fifty 



m Watts, 'Dictionary of Chemistry,' vol. ii. pp. 936, 937. 

 11 ' Discovery of Subterranenl Treasure,' London, 1639, P- 4^- 



