XXVII.] GENERALISATION. 611 



eiTor. It is truly difficult at the first moment to recognise 

 any similarity between the gradual rusting of a piece of 

 iron, and the rapid combustion of a heap of straw. Yet 

 Lavoisier's chemical theory was founded upon the similarity 

 of the oxydising process in one case and the other. "We 

 have only to divide the iron into excessively small particles 

 to discover that it is really the more combustible of the 

 two, and that it actually takes fire spontaneously and burns 

 like tinder. It is the excessive slowness of the process in 

 the case of a massive piece of iron which disguises its real 

 character. 



If Xenophon reports truly, Socrates was misled by not 

 making sufficient allowance for extreme differences of de- 

 gree and quantity. Anaxagoras held that the sun is a fire, 

 but Socrates rejected this opinion, on the ground that we 

 can look at a fire, but not at the sun, and that plants grow 

 by sunshine Avhile they are killed by fire. He also pointed 

 out that a stone heated in a fire is not luminous, and soon 

 cools, whereas the sun ever remains equally luminous and 

 hot.^ All such mistakes evidently arise from not perceiv- 

 ing that difference of quantity may be so extreme as to 

 assume the appearance of difference of quality. It is the 

 least creditalile thing we know of Socrates, that after point-- 

 ing out these supposed mistakes of earlier philosophers, he 

 advised his followers not to study astronomy. 



Masses of matter of very different size may be expected 

 to exhibit apparent differences of conduct, arising from the 

 various intensity of the forces brought into play. Many 

 persons have thought it requisite to imagine occult forces 

 producing the suspension of the clouds, and there have even 

 lieen absurd theories representing cloud particles as minute 

 water-balloons buoyed up by the warm air within them. 

 But we have only to take proper account of the enormous 

 comparative resistance which the air opposes to the fall of 

 minute particles, to see that all cloud particles are probably 

 constantly falling through the air, but so slowly that there 

 is no apparent effect. Minei'al matter again is always re- 

 garded as inert and incapable of spontaneous movement. 

 We are struck by astonishment on observing in a powerful 

 microscope, that every kind of solid matter suspended in 



' Memorabilia, iv. 7. 



R K 2 



