six.] EXPERIMENT. 419 



cannot remove except by accident. These difficulties we 

 will shortly consider in succession. 



It is beautiful to observe how the alteration of a single 

 circumstance sometimes conclusively explains a pheno- 

 menon. An instance is found in Faraday's investigation 

 of the behaviour of Lycopodium spores scattered on a 

 vibrating plate. It was observed that these minute spores 

 collected together at the points of greatest motion, whereas 

 sand and all heavy particles collected at the nodes, where 

 the motion was least. It happily occurred to Faraday to 

 try the experiment in the exhausted receiver of an air- 

 pump, and it was then found that the light powder behaved 

 exactly like heavy powder. A conclusive proof was thus 

 obtained that the presence of air was the condition of im- 

 portance, doubtless because it was thrown into eddies by 

 the motion of the plate, and carried the Lycopodium to 

 the points of greatest agitation. Sand was too heavy to be 

 carried by the air. 



Exclusion of Indifferent Circumstances. 



From what has been already said it will be apparent 

 that the detection and exclusion of indifferent circum- 

 stances is a work of importance, because it allows the 

 concentration of attention upon circumstances which con- 

 tain the principal condition. Many beautiful instances may 

 be given where all the most obvious antecedents have been 

 shown to have no part in the production of a phenomenon. 

 A person might suppose that the peculiar colours of mother- 

 of-pearl were due to the chemical qualities of the substance 

 Much trouble might have been spent in following out that 

 notion by comparing the chemical qualities of various iri- 

 descent substances. But Brewster accidentally took an 

 impression from a piece of mother-of-pearl in a cement of 

 resin and bees'-wax, and finding the colours repeated upon 

 the surface of the wax, he proceeded to take other impres- 

 sions in balsam, fusible metal, lead, gum arabic, isinglass, 

 &c., and always found the iridescent colours the same. He 

 thus proved that the chemical nature of the substance is a 

 matter of indifference, and that the form of the surface is 

 the real condition of such colours. 1 Nearly the same may 

 1 Treatise on Optics, by Brewster, Cab. Cyclo. p. 117. 



