26 THE PRINCIPLES OF SCIENCE. 



sand and all heavy particles collect at the nodes, where 

 motion is least. But 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 con- 

 dition of importance, doubtless because it was thrown into 

 eddies by the motion of the plate, and thus carried the 

 Lycopodium to the points of greatest agitation. Sand 

 was too heavy to be thus carried by the air. 



Exclusion of Indifferent Circumstances. 



From what has been already said it will be apparent that 

 in the investigation of any new phenomenon the detection 

 and exclusion of indifferent circumstances is a work of 

 great importance, because it allows the concentration of 

 attention upon circumstances which may contain the 

 principal condition. There will always be a multitude 

 of things which we are only too ready to neglect, but 

 many beautiful instances may be given where all the most 

 obvious circumstances have been shown to have no part in 

 the production of a phenomenon. Every person would 

 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 iridescent 

 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, proceeded to take other impressions 

 in balsam, fusible metal, lead, gum arabic, isinglass, &c., 

 and always found the iridescent colours the same. He 

 thus proved that the chemical nature is wholly a matter 



