EXCEPTIONAL PlIESOlfKXA. 333 



belonging to any part of the spectrum, and it seems pro- 

 bable that this effect arises from the impact of distinct but 

 conflicting atoms. Nor is it in light only that we discover 

 limiting exceptions to the law of forced vibrations ; for if 

 we closely observe gentle waves lapping upon the stones 

 at the edge of a lake or other piece of water, we shall 

 notice that each larger wave in breaking upon a stone 

 gives rise to a series of waves of a smaller order. Thus 

 there must be constantly in progress a degradation in the 

 magnitude of water-waves. The principle of forced vibra- 

 tions seems then to be too generally stated by Sir John 

 Herschel, but it must be a very difficult question of me- 

 chanical theory to discriminate the circumstances in which 

 it does and does not hold true. 



We may sometimes foresee the possible existence of 

 exceptions yet unknown by experience, and limit the 

 statement of our discoveries accordingly. Veiy extensive 

 inquiries have shown that all substances yet examined 

 fall into one of two classes ; they are all either ferro- 

 magnetic, that is, magnetic in the same way as iron, or 

 they are diamagnetic like bismuth. But it does not 

 thence foUow that every substance must be ferro-magnetic 

 or diamagnetic. The magnetic properties are shown by 

 Sir W. Thomson to depend upon the specific inductive 

 capacities of the substance in three rectangular directions. 

 If these inductive capacities are all positive, we have a 

 ferro-magnetic substance ; if negative, a diamagnetic sub- 

 stance ; but if the specific inductive capacity were posi- 

 tive in one direction and negative in the others, we should 

 have an exception to previous experience, and could not 

 place the substance under either of the present recognised 

 classes. 



So many gases have been reduced to the liquid state, 

 and so many solids fused, that scientific men rather hastily 



' Philosophical Magazine,' 4th Series, vol. i. p. 182. 



