314 Pr. G. Gore on Changes of Temperature caused 



hire with that of amount of dissolved substance abstracted by 

 the powder (ibid) , we find that the two do not usually vary 

 together. This is a result which might be expected if the 

 amount of change of temperature depends upon all the con- 

 ditions above suggested, and especially as it further depends 

 in many cases upon heat absorbed by contact of the powder 

 with the dissolved substance. 



The fact that the portion of dissolved substance which 

 unites with the powder is entirely removable by washing with 

 water proves the reversibility of the action, and that the 

 phenomena are essentially the same as those of chemical urn on 

 and separation in reversible cases (ibid.). The substitution 

 also of a film of water or solution for oie of air in contact 

 with the powder must obey the same general laws as that of 

 substitution in recognized chemical actions. And we may 

 further reasonably infer that whilst the union of the water or 

 of the dissolved substance v\lth the powder is usually attended 

 by evolution of beat, the separation of the film of air from 

 the powder is frequently accompanied by absorption of heat. 



As the action is purely a surface one, and the amount of 

 rise of temperature increases with that of immersed solid 

 surface, it is essentially the same whether the amount of sur- 

 face is obtained by means of large particles or small ones. 



As the union of the water or the dissolved substance with 

 the powder is attended by loss of molecular energy, the action 

 must be due to both of the touching substances, and must 

 obey the law of equality of action and reaction, a portion of 

 the molecular motion of each substance being neutralized and 

 changed into heat, and the molecules of the liquid which have 

 loso motion must remain nearer to the solid, or, in ordinary 

 terms, " adhering" to it. The amount of heat set free by the 

 11 adhesion " of the liquid to the solid is probably dependent 

 both upon the mass and the loss of velocity of the " adhering" 

 molecules. As nearly the whole of the change of temperature 

 caused by the " adhesion " occurred in a few minutes, the 

 mutual loss of molecular energy by the two substances must 

 have quickly ceased. After the evolution of heat has ceased 

 there remain permanent changes of superficial molecular state 

 and a new condition of balance between the different sets of 

 molecules present, until by washing and subsequent heating 

 the powder is restored to its original state. 



The reversibility of the action, and the new state of mole- 

 cular balance intervening between the evolution of heat by 

 wetting the powder and the absorption of heat by drying it, 

 is analogous to cases of reversible chemical union, to the 

 sudden electric charge of two different metals by mere con- 



