EXPERIMENT. 43 



duction of sounds, evidently because their vacuum was not 

 sufficiently perfect 11 . Otto von Guericke fell into- a like 

 mistake in the use of his newly-constructed air-pump, 

 doubtless from the unsuspected presence of air sufficiently 

 dense to convey the sound of the bell 1 . 



It is hardly requisite to point out that the doctrine of 

 spontaneous generation is due to the unsuspected presence 

 of germs, even after the most careful efforts to exclude 

 them k , and in the case of many diseases, both of animals 

 arid plants, germs which we have no means as yet of de- 

 tecting and examining, are doubtless the active cause. It 

 has long been a subject of dispute, again, whether the 

 plants which spring up from newly turned land grow 

 from seeds long buried in that land, or from seeds brought 

 by the wind. Argument is unphilosophical when direct 

 trial can readily be applied ; for by turning up some old 

 ground, and covering a portion of it with a glass case, the 

 conveyance of seeds by the wind can be entirely prevented, 

 and if the same plants appear within and without the 

 case, it will become clear that the seeds are in the earth. 

 By gross oversight some experimenters have thought 

 before now that crops of rye had sprung up where oats 

 had been sown 1 . 



Blind or Test Experiments. 



Every correct and conclusive experiment necessarily 

 consists in the comparison of results between two different 

 combinations of circumstances. To give a fair probability 



h ' Essayes of Natural Experiments,' &c. Englished by Richard Waller, 

 p. 50. 



1 Whewell, 'History of the Inductive Sciences,' 3rd edition, vol. ii. 

 p. 246. 



k Berkeley's 'Introduction to Cryptogamic Botany,' pp. 258, 259. 



1 Dr. Weissenborn, in the new series of * Magazine of Natural History,' 

 vol. i. p. 574, quoted in 'Vestiges of Creation/ 2nd edition, p. 222. 



