434 THE PRINCIPLES OF SCIENCE. [CHAP 



Experience is ultimately the basis of all our inferences, 

 but if we can bring immediate experience to bear upon the 

 point in question we should not trust to anything more 

 remote and liable to error. When Faraday examined the 

 magnetic properties of the bearing apparatus, in the absence 

 of the substance to be experimented on, he really made a 

 blind experiment (p. 431). 



We ought, also, to test the accuracy of a method of ex- 

 periment whenever we can, by introducing known amounts 

 of the substance or force to be detected. A new analytical 

 process for the quantitative estimation of an element 

 should be tested by performing it upon a mixture com- 

 pounded 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. 1 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 known to exist. 2 



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 a 

 magnitude inappreciable to our senses, or confounded with 

 other effects of much greater amount. As there is no 

 limit on d priori grounds to the smallness of a phenome- 

 non, we cati never, by a single experiment, 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 



1 Jevons in Watts' Dictionary of Chemistry, vol. ii. pp. 936, 937. 

 '" Discovery of Subterranecd Treasure.. London, 1639, p. 48. 



