214 ANNUAL OF SCIENTIFIC DISCOVERY. 



and luminous bodies, and which are all produced in the same manner, viz., 

 by undulations, but of different lengths, those of the greatest lengths 

 having a heating, but not an illuminating, power ; those of less lengths a 

 luminiferous property also ; those of still less lengths, little heating, but 

 higher chemical power. The arguments of M. Mellonl were specially 

 dwelt upon ; as also the striking confirmation derived from certain calcu- 

 lations founded on the wave theory, which assigned a limit to all refrac- 

 tion, according closely with that found experimentally for heat in rock 

 salt. 



In the discussion which followed, Prof. Stevelly begged to ask Prof. 

 Powell and Prof. Thompson how, in their opinion, the expansion of 

 bodies by increased extent of the vibrations or on the Dynamical theory 

 increased heat was to be reconciled with the well-known fact, that four 

 substances water, antimony, cast-iron and bismuth were known to 

 expand in the process of cooling. Prof. Powell replied, that these cases 

 alluded to by Prof. Stevelly constituted a difficulty in any known theory 

 of heat, and were therefore not more adverse to the Dynamical theory than 

 to any other. Prof. Thomson said, that besides those cases stated by Prof. 

 Stevelly being no greater difficulty in the Dynamic theory than in any 

 other, that theory seemed to hold out a hope of explaining these anoma- 

 lies. In his opinion, water while cooling, for instance, began at its maxi- 

 mum density to approach that molecular arrangement which it fully and 

 fixedly attained in the act of solidifying. Prof. Stevelly said he was not 

 satisfied with the answer, that this was a difficulty in all other theories as 

 well as in this, for a true theory must be at least reconcilable with all 

 well-established facts ; and as he was nearly satisfied that this was a true 

 theory, and as the facts he had stated were indisputable, he was sure the 

 Section would feel grateful to Prof. Thomson if he would explain. Sup- 

 posing each molecule to have poles, he conceived those facts might meet a 

 physical explanation by the Dynamical theory. Prof. Thomson, in an- 

 swer to Prof. Stevelly's question, if the Dynamical theory of heat, on 

 which the new theory of solar heat is founded, could explain the strange, 

 almost anomalous, expansion which water exhibits before freezing, 

 remarked that, since in the act of freezing water expands, it is certain that 

 the polar condition which the particles assume fixedly when solid is such 

 that they keep one another farther asunder then than when turning about 

 into all possible relative positions, as they probably do in their thermal 

 motions when the mass is liquid and warm. It appears, then, very highly 

 probable that, when the water is cooled towards the freezing point, the 

 energy of these thermal motions is diminished, so that the excursions of 

 the polar axes of the particles become confined to narrower and narrower 

 limits, and the axes of contiguous particles begin to affect or tend towards 

 those relative positions into which they settle in freezing. This tendency 

 would make the particles begin to keep one another farther asunder, or 

 make the whole mass expand, even before it freezes, when its temperature 

 is lowered below a certain limit, and would explain the fact that water 



