THE USE OF HYPOTHESIS. 153 



Although the Science of Light presents us with the 

 most beautiful examples of crucial experiments and ob- 

 servations, instances are not wanting in other branches of 

 science. Copernicus asserted in opposition to the ancient 

 Ptolemaic theory that the earth and planets moved round 

 the sun, and he predicted that if ever the sense of sight 

 could be rendered sufficiently acute and powerful, we 

 should see phases in Mercury and Venus. Galileo with 

 his telescope was able, in 1610, to verify the prediction as 

 regards Venus, and subsequent observations of Mercury 

 lead to a like conclusion. The discovery of the aberra- 

 tion of light added a new proof, still further strengthened 

 by the more recent determination of the parallax of fixed 

 stars. Hooke proposed to prove the existence of the 

 earth's diurnal motion by observing the deviation of a 

 falling body, an experiment successfully accomplished by 

 Benzenberg ; and Foucault's pendulum has since fur- 

 nished an additional indication of the same motion, which 

 is indeed also apparent in the direction of the trade winds. 

 All these are crucial facts in favour of the Copernican 

 theory. 



Davy's discovery of potassium and sodium in 1 807 was 

 a good instance of a crucial experiment ; for it decisively 

 confirmed Lavoisier's views, and at the same time nega- 

 tived the ancient notions of phlogiston. 



Descriptive Hypotheses. 



There are some, or probably many, hypotheses which 

 we may call descriptive hypotheses, and which serve for 

 little else than to furnish convenient names. When a 

 certain phenomenon is of an unusual and mysterious kind, 

 we cannot even speak of it without using some analogy. 

 Every word implies some resemblance between the thing 

 to which it is applied, and some other thing, which fixes 



