catcd through an apparent vacuum by the undulations 

 of a very subtile elastic medium, which is also concerned 

 in the phenomena of light." 



Our experimental labours and our mathematical 

 investigations have considerably advanced our know- 

 ledge since the time of Newton ; yet still each theory of 

 heat strangely resembles the mystic lamp which the 

 Rosicrucian regarded as a type of eternal life a dim 

 and flickering symbol, in the tongue-like flame of which 

 imagination, like a child, can conjure many shapes. 



Modern theory regards heat as a manifestation of 

 motion, and experiment proves that a body falling 

 through a certain space generates a definite quantity of 

 heat, while observation shows that the waters at the 

 base of the Falls of Niagara possess a temperature 1 

 higher than when they first glide over the edge of the 

 precipice. 



This increase of temperature is due to the mechanical 

 force due to the fall, and is no more an evidence of the 

 conversion of motion into heat, than is the old experi- 

 ment of rubbing a button until it becomes hot. At all 

 events, the fact that a given amount of mechanical force 

 always produces an equivalent of heat is as applicable to 

 the idea of a " subtile elastic medium" which is diffused 

 through all matter, as to the, at present, favourite hypo- 

 thesis. 



So far has this view been strained, that the tempera- 

 ture of the planets has been referred to their motions, 

 and speculation has aided the mathematician in deter- 

 mining the cessation of planetary motion, by the conver- 

 sion of it into heat. It is true that other theorists have 

 supposed points in space upon which this heat might be 

 concentrated and reflected back again to produce motion. 



There may be much of the poetic element in such 

 speculations, but it is of that order which belongs rather 

 -to the romantic than to the real. 



A speculation which has more of truth, and which is, 



