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



selenium, and even oxides of ammonium, which it remains 

 for chemists to verify. 1 



Prediction ly Inversion of Caiise and Effect. 



There is one process of experiment which has so often led 

 to important discoveries as to deserve separate illustra- 

 tion I mean the inversion of Cause and Effect. Thus if 

 A and B in one experiment 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 force, cold is the result; that is, B (air) and C 

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

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

 Similar results may be expected in a multitude of cases. 

 It is a familiar law that heat 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 

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

 with his usual skill. 2 



This inversion of cause and effect in the case of heat 

 may be itself inverted in a highly curious manner. It 

 happens 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 being stretched it grows slightly warm, and when 

 relaxed cold. 



The reader will see that some of the scientific predictions 

 mentioned in preceding sections were due to the principle 



1 Hofrnaim's Introduction to Chemistry, pp. 224, 225. 



2 Philosophical Transactions (1855), voL cxlv. pp. 100, &c. 



N N 



