XXIV.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 545 



Felenium, and even oxides of aunnoniun), which it remains 

 for chemists to verify.^ 



Prediction hy diversion of Cause and Effect. 



There is one process of experiment which lias so often led 

 1o important discoveries as to deserve separate illustra- 

 tion — I mean the inversion of Cause and Effect. Thus ii 

 A and V> in one ex])eriiiient produce C as a consequent, 

 then antecedents of the nature of B and C may usually be 

 made to produce a consequent of the nature of A inverted 

 in direction. When we apply heat to a gas it tends to 

 expand ; hence if we allow the gas to expand by its own 

 elastic fome, cold is the result ; that is, B (air) and C 

 (expansion) produce the negative of A (heat). Again, B 

 (air) and con)pression, the negative of C, produce A (heat). 

 iSimilar results may be expected in a multitude of cases. 

 Tt is a iamiliar law that lieat expands iron, What may be 

 expected, then, if instead of increasing the length of an 

 iron bar by heat we use mechanical force and stretch the 

 bar? Having the bar and the former consequent, expan- 

 sion, we should expect the negative of the former anteced- 

 ent, namely cold. The truth of this inference was proved 

 bv Dr. Joule, who investigated the amount of the effect 

 with his usual skill. ^ 



This inversion of cause and effect in the case of heat 

 may be itself inverted in a highly curious' manner. It 

 Jiappens that there are a few substances which are unex- 

 plained exceptions to the general law of expansion by heat. 

 India-rubber especially is remarkable for contracting when 

 heated. Since, then, iron and india-rubber are oppositely 

 related to heat, we may expect that as distension of the 

 iron produced cold, distension of the india-rubber will 

 produce heat. This is actually found to be the case, and 

 anyone may detect the effect by suddenly stretching an 

 india-rubber band while the middle part is in the mouth. 

 When Ijeing stretched it grows slightly warm, and when 

 relaxed cold. 



The reader will see that some of the scientific predictions 

 mentioned in {ireceding sections were due to the principle 



J Hofniaim's Introduction to Chemistry, pp. 224, 225. 



* J'hilcsojjhical Transactions (1855), vol. cxlv. pp. 100, &c. 



N N 



