30 DISSERTATION SECOND. [part i. 



pernican system, that is, either on the system which makes 

 the Earth, or that which makes the Sun, the centre of the 

 planetary motions, a cautious philosopher would hesitate 

 about which of the two he should adopt, and notwithstand- 

 ing that one of them was recommended by its superiour 

 simplicity, he might not think himself authorized to give 

 to it a decided preference above the other. If, however, 

 he consider the motion of these bodies in latitude, that is to 

 say, their digressions from the plane of the ecliptick, he 

 will find a set of phenomena which cannot be reconciled 

 with the supposition that the earth is the centre of the pla- 

 netary motions, but which receive the most simple and 

 satisfactory explanation from supposing that the sun is at 

 rest, and is the centre of those motions. The latter phe- 

 nomena would therefore serve as instantiae cruris, by 

 which the superiour credibility of the Copernican system 

 was fully evinced. 



Another example which I shall give of an instantia cru- 

 ris is taken from chemistry, and is, indeed, one of the most 

 remarkable experiments which has been made in that 

 science. 



It is a general fact observed in chemistry, that metals 

 are always rendered heavier by calcination. When a mass 

 of tin or lead, for instance, is calcined in the fire, though 

 every precaution is taken to prevent any addition from the 

 adhesion of ashes, coals, &c. the absolute weight of the 

 mass is always found to be increased. It was long before 

 the cause of this phenomenon was understood. There 

 might be some heavy substance added, though what it was 

 could not easily be imagined ; or some substance might 

 have escaped, which was in its nature light, and possessed 

 a tendency upwards. Other phenomena, into the nature of 

 which it is at present unnecessary to inquire, induced 

 chemists to suppose, that in calcination a certain substance 



