1904] Cotton ~by Water and % Water Vapour. 231 



much smaller rise, and in its case the glistening appearance is absent. 

 Moreover, the quantity of water which penetrates the cotton 

 wrapping and remains mechanically adherent to it and the thermo- 

 meter when they are lifted out after, say, J hour's immersion, 

 may be determined by weighing, and it is found to be less than the 

 weight of the dry cotton in the case of the ordinary material, but 

 several times greater in the case of the absorbent variety. Obviously, 

 therefore, when they are immersed, the former is both better insulated 

 and has the smaller heat capacity, and will thus show a larger 

 rise of temperature for a given heat production, and the observed 

 difference does not necessarily indicate that the absorbent wool is 

 inherently less susceptible to such action as may be the cause of the 

 phenomenon. 



The true nature of this action is indicated in the first place by the 

 >bservation that immersion in water is not essential, for the covered 



lermometer behaves in a precisely similar manner when exposed to 

 dr saturated with water vapour. The rise of temperature is as great 

 >r greater, though it may take 5 or 6 minutes to reach the maximum 

 stead of 2 or 3, and the subsequent slow cooling curve is similar, 

 well as the effects of varying conditions. In fact it has been proved 

 )y the experiments to be described that the whole action is essentially 

 the same, whether a bath of water or of saturated air be employed, 



id that in both cases the heat production is due, at all events, 

 )rimarily, to the condensation of water vapour on the surface of the 



>tton fibres. More strictly, this is proved in the case of saturated 



ir immersion, and its extension to the other case is necessitated by 

 the proof of the complete similarity of the thermal changes. In the 

 of immersion in water the vapour which condenses on the cotton 



mst be produced by evaporation from the liquid, which is prevented 

 from complete contact with the fibres by the air which adheres to 



lem and fills the interstices. There is, therefore, an automatic dis- 

 }illation from the water on to the cotton which acts as a condenser, 



id the heat gained there must be matched by an equal loss of heat 

 by the adjacent water, though the large heat capacity of the latter 

 prevents any fall of temperature comparable with the observed rise. 

 "Absorbent 1 " cotton wool is found to behave in saturated air exactly 

 like the ordinary variety, in contrast with its much smaller tempera- 

 ture effect in water. This is in complete accord with the theory. 

 The condensation of vapour that occurs during exposure to saturated 

 air never renders the cotton sensibly moist, though it may absorb over 

 20 per cent, of its weight in a few hours. 



It has long been known that rise of temperature results from the 

 immersion of finely divided solids in water and other liquids. 

 Pouillet* examined a large number of substances, both inorganic and 

 * ' Ann. de Chim. et de Phys.,' 1822, (2), vol. 20, pp. 141162. 



