DISCO VEKY BY INVESTIGATING NEGLECTED TRUTHS. 487 



taring and other operations ; examining exceptional, ex- 

 treme, and conspicuous instances, common but neglected 

 circumstances, peculiar minerals, rare substances, residues 

 of manufacturing processes, the ashes of peculiar or rare 

 plants and animals, &c. 



a. By examining neglected truths and hypotheses. 

 Important facts and hypotheses are sometimes neg- 

 lected for many years, until circumstances arise to call 

 attention to them. The facts discovered by Greber (who 

 was born in the year 830), that iron, lead, and copper 

 became heavier by being heated to redness and cooled in 

 the air, so as to become oxidised ; and by Boyle (during 

 the seventeenth century), that tin behaved similarly, re- 

 mained almost unnoticed, or at any rate uninvestigated 

 and without a true interpretation, until about the year 

 1778, when Lavoisier inferred their true explanation, and, 

 by means of them and similar experiments of his own, 

 made the great discovery of the true nature of oxidation, 

 combustion, respiration, and of chemical union in general. 

 Avogadro's hypothesis, published in 1811, and reproduced 

 by Ampere in 1814, asserting that equal volumes of all 

 gases contain equal numbers of molecules, was also neg- 

 lected for a long time, but has since been proved by experi- 

 ment to be one of the greatest truths of chemical science. 



b. By examining peculiar or unexplained truths in 

 science. Galileo, at the age of nineteen, in the year 1583, 

 observing that a lamp, suspended from the roof of the 

 cathedral of Pisa, took the same time to swing backwards 

 and forwards whether the arc of vibration was more or 

 less, investigated the circumstance, and found the prin- 

 ciple of the pendulum, viz., that the period of vibration 

 was constant, provided that the length of the string re- 

 mained the same and that the arc of vibration was not 

 very large. In the year 1589 he further observed that 



