216 Dr. H. Hndson on Wave-Theories of 



In this case it is obvious we cannot have recourse to a ^^ theory 

 of exchanges j '' and yet the explanation of the phenomena must, 

 I conceive, on the undulatory theory be similar for both light 

 and heat. I may add that a diminution of the index of refrac- 

 tion produced by increasing the temperature of a medium (as 

 shown experimentally by Gladstone, Dale, and Landolt) appears 

 an obvious consequence of the views here indicated. 



Whether we consider the case of sound, light, or heat, there 

 can be no doubt that ^' intensity '' depends on the amplitude of 

 vibration, and quality (viz. pitch in music, and colour or refran- 

 gibility in light and heat) depends on the number of vibrations 

 in a given time. Increased temperature thus represents greater 

 amplitude of the vibrations of the molecules of matter as con- 

 tradistinguished from those of their associated sether, which, 

 however, we must consider as generally synchronizing with the 

 former, although their num.ber per second must be enormously 

 greater. 



When we consider molecular motions separately, we can 

 scarcely doubt that the momenta of the molecules (V x 

 W) of two bodies at the same temperature are equal; that is 

 to say, the ^^ velocity of vibration''^ (in chemical language '^ the 

 capacity for heat^^) varies inversely as the molecular weight — a 

 deduction which is in accordance with experiment. 



A curious consequence appears to flow from these views when 

 we compare the same substance at the same temperature in dif~ 

 ferent states of aggregation. Thus water and ice at zero Centi- 

 grade have the same molecular momentum. But the specific 

 heat of ice is only one half that of water (viz. as 0*5050 : 1), 

 therefore the molecule of ice should be to that of water as 2 : 1 

 (to maintain their equal momenta) . Again, in comparing water 

 and steam (at the boiling-point), their specific heats (as 1 : 0*4805 

 per Regnault) apjjear in like manner to indicate a ratio of 2 : 1, 

 whence it would in fact foUovf that the molecular weight of 

 water is only one half of the molecular weight of either ice or 

 steam ! Thus if the molecular weight of steam be taken as 9 

 (in accordance with hydrogen^s gaseous unit), then the molecule 

 of water can only be 4*5, and the atomic weights of hydrogen 

 and of oxygen cannot exceed 0*25 and 4 respectively ; so that the 

 gaseous molecule of both hydrogen and oxygen must consist of 

 at least 4 atoms each ! 



With regard to electrical phenomena, it appears to be abun- 

 dantly proved by Faraday that the two electricities exist in equal 

 quantities associated with the atoms or molecules of matter, and 

 the amount is so great that the electricity evolved from \\ grain 

 of water (decomposed) is adequate to charge 500 acres of cloud 

 surface; they are each self-repellent, and are both attracted by 



