254 On the Wetting of Cotton by Water and by Water Vapour. 



value of his heat production as about O'OOl cal. per sq. cm. of 

 surface of exposed glass or silica, whereas it can be shown from facts 

 already given that even in the case of saturated air immersion, about 

 twelve times this quantity of heat is not generated but transferred per 

 square centimetre of cotton wool in the first few minutes when, as just 

 stated, only a small fraction of the total action has occurred. As the Parks 

 effect is so small it is easy to understand how it escaped observation in 

 the author's experiment with glass wool, already described, where the 

 conditions were altogether unfavourable, having been arranged with a 

 different object ; for Parks himself, using a larger quantity of material, 

 which he gently stirred with about the same amount of water as 

 was contained in the author's immersion vessel, obtained a rise of 

 only 0'01. 



It is difficult to say for certain whether the heating effects observed 

 originally by Pouillet were due to the phenomenon investigated by 

 Parks and styled by him " the Pouillet effect," or were the results of 

 action similar to that described in this paper, or included both. But 

 as he heaped his dry solid round the bulb of a delicate thermometer, 

 and was careful not to disturb the heap by stirring it after adding the 

 liquid, it is unlikely that true contact was established at once and 

 completely, and it seems reasonable to suppose that he had to do with 

 local heating by condensation of vapour. The still more striking, but 

 similar observations of Martini* are probably to be explained in 

 the same way and not, as he suggests, by the solidification of liquid 

 water. 



In conclusion, it may be pointed out that the hygroscopic behaviour 

 of cellulose and the thermal changes associated with it have a practical 

 importance of their own on the side of vegetable physiology, and 

 perhaps also in other directions, apart from any interest they may have 

 on purely physical grounds. 



* ' Phil. Mag.' [5] vol. 47, p. 329; vol. 50, p. 618; [6] vol. 5, p. 595. 



