THERMOTICS. 601< 



difficult to measure all the heat gained or lost in any of the changes 

 here contemplated. That friction, agitation of fluids, condensation of 

 gases, conversion of gases into fluids and liquids into solids, produce 

 heat, is undoubted : and that the quantity of such heat may be mea- 

 sured by the mechanical force which produces it, or which it produces, 

 is a generalization which will very likely be found a fertile source of 

 new propositions, and probably of important consequences. 



As an example of the conclusions which Professor Thomson draws 

 from this doctrine of the mutual conversion of motion and heat, I may 

 mention his speculations concerning the cause which produces and sus- 

 tains the heat of the sun. 1 He conceives that the support of the solar 

 heat must be meteoric matter which is perpetually falling towards the 

 globe of the sun, and has its motion converted into heat. He inclines 

 to think that the meteors containing the stores of energy for future 

 Sun-light must be principally within the earth's orbit ; and that we 

 actually see them there as the " Zodiacal Light," an illuminated shower. 

 or rather tornado, of stones. The inner parts of this tornado are 

 always getting caught in the Sun's atmosphere, and drawn to his 

 by gravitation. 



1 On the Mechanical Energies of the Solar System. Edinb. Trans, vol. xxi. 

 part L (1854), p. 67. 



VOL. II 39 



