by Contact of Liquids with Powdered Silica fyc. 313 



In all the experiments the formation of a liquid film evolved 

 a larger amount of heat than that absorbed by the air-film 

 which it displaced. 



The further fact must not be overlooked, that in all these 

 experiments there existed a minute source of error which 

 could not be avoided, viz., that although each powder em- 

 ployed appeared to be perfectly wetted in ten or twenty 

 seconds, it was not so, but continued to evolve a very small 

 quantity of minute bubbles of air after that period. With 

 peroxide of iron this action ceased in about half an hour, and 

 with silica and alumina it continued during several days ; but 

 even in the latter cases it was probably not sufficient to affect 

 the general result to an extent of a one-hundredth part. 



As all the observations of maximum rise of temperature 

 were compelled to be taken at the moment they occurred, 

 viz., in about three minutes after mixing, any amount of heat 

 evolved after that period could not be included. Even when 

 a separate portion of the powder was thoroughly shaken with 

 some of the liquid in a bottle, the bubbles similarly appeared, 

 but with coarse white sand they did not occur. Another 

 circumstance (which has already been mentioned) is that the 

 temperature of the immersed powder was higher than that of 

 the liquid above it. 



The general results confirm the conclusion arrived at by 

 previous investigators (Pouillet and Meissner), that in all 

 known cases the contact of a liquid with an insoluble powder 

 evolves heat. From the present ones, however, in which 

 organic and oxidizable substances were avoided and only 

 water and aqueous solutions were employed, it appears prob- 

 able that (subject to the foregoing considerations), whilst 

 many dissolved substances evolve heat and add to that pro- 

 duced by the water, a large number absorb heat and subtract 

 from that effect : with silica nearly half the number of solu- 

 tions evolved less than water alone. 



In all aqueous solutions the change of temperature appears 

 to be attributable to several causes, viz. : — 1st, the union of 

 the water with the powder ; 2nd, that of the dissolved sub- 

 stance with the powder ; and 3rd, the separation of the film 

 of air ; and the rise of temperature is reasonably explained 

 by the general view that a degree of mechanical union takes 

 place of the powder with the water or with the dissolved 

 substance, the powder abstracting from the liquid, either some 

 of the water or of the dissolved substance or of both (see 

 " Decomposition of Liquids by Contact with Powdered Silica," 

 Proc. Birm. Phil. Soc. 1894, vol. ix. pp. 1-24). 



If we compare the order of magnitude of rise of tempera- 



