EXCEPTIONAL PHENOMENA. 313 



forms are similarly modified, so as to produce a series of 

 what are called hemihedral forms. 



Apparent but Congruent Exceptions. 



Not unfrequently a law of nature will present results 

 in certain circumstances which appear to be entirely in 

 conflict with the law itself. Not only may the action of 

 the law be much complicated and disguised, but it may 

 in various ways be reversed or inverted, so that all care- 

 less observers are misled. Ancient philosophers gene- 

 rally believed that while some bodies were heavy by 

 nature, others, such as flame, smoke, bubbles, clouds, &c., 

 were essentially light, or possessed a tendency to move 

 upwards. So acute and learned an inquirer as Aristotle 

 entirely failed to perceive the true nature of buoyancy or 

 apparent lightness, and the doctrine of intrinsic lightness, 

 being expounded in his works, became the accepted view 

 for many centuries. It is true that Lucretius was fully 

 aware why flame tends to rise, holding that 



' The flame has weight, though highly rare, 

 Nor mounts but when compelled by heavier air.' 



Archimedes also was so perfectly acquainted with the 

 buoyancy of bodies immersed in water, that he could not 

 fail to perceive the existence of a parallel effect in air. 

 Yet throughout the early middle ages the light of true 

 science, clear though feeble, could not contend with the 

 powerful but confused glare of the false Peripatetic doc- 

 trine. The genius of Galileo and Newton was required 

 to convince people of the simple truth that all matter 

 is heavy, but that the gravity of one substance may be 

 overborne by that of another, as one scale of a balance 

 is carried up by the preponderating weight in the oppo- 

 site scale. It is curious to find Newton gravely explaining 



