14 Mr. J. J. Water ston on Capillarity 



to give the top of the liquid a convex form ; like mercury in glass 

 and in capillary tubes the liquid should descend beyond the outer 

 level. 



This was actually remarked by M. Wolf {Ann. de Chim. p. 270); 

 I have also observed the convexity. It quickly assumes a sugar- 

 loaf aspect. It requires a steady temperature to show the sur- 

 face a distinct line. 



]\I. Wolf further observed, that the rate at which the capillary 

 column of aether descended, increased at the higher temperatures 

 (p. 271). 



It seems to be almost certain that the rate at which latent 

 heat diminishes also augments with the temperature. The fol- 

 lowing is the evidence on this point. 



M. Rcgnault's empirical formula for the latent heat of 

 aqueous vapour is \_^ = 606-5— 0-695if (Centigrade). If the 

 ratio -695 applies to alcohol, which it does nearly according to 

 M.Despretz,thenX„ = 209-(^-78)(0-695)(0-64) = 244-0-45^, 

 in wliich 209° is the latent heat of alcohol at the boiling-point, 

 and 0'64 its specific heat, according to ?.IM. Fabre and Silber- 

 man's observations [Ann. de Chim. vol. xxxvii. p. 466). If the 

 ratio 0'45 did not increase, transition could not take place until 

 542° C, instead of 260° or 280° C. 



That the specific heat increases with the temperature, is proved 

 by the quotient of the latent heat, by the specific heat being 

 about 400° C, showing that the quantity of heat required to boil 

 oflF a measure of alcohol is sufficient to raise it to 400° C. if its 

 specific heat were constant. 



The same remark applies to water ; its latent heat alone would 

 carry it far beyond the point of transition. But it is otherwise 

 with icthcr, if \vc take MM. Fabre and Silbcrmau's data. The 

 latent heat could only raise it to 220° C, whereas transition 

 takes jjlace at 200°. The difference, even including the proba- 

 ble amount of sensible heat, gives little room for increase in the 

 specific heat, or for acceleration in the diminution of latent heat. 



The same remark applies to turpentine, which I found to 

 undergo a chemical change before it reached the point of trans- 

 ition. It is probable that a portion of the latent heat is absorbed 

 in partially overcoming the chemical forces that keep the elements 

 of each compound molecule together. Thus we have nitrous, formic 

 and acetic jcthcrs boiling at low temperatures, and rising with 

 the fcther separated from the acid. Oxalic and succinic asthers, 

 on the other hand, boil at high temperatures, and rise unchanged, 

 the molecule of sulphuric tether still clinging to the acid in the 

 state of vapour. Even with regard to such permanent liquids as 

 water, we must be prepared to view as possible a certain absorp- 

 tion of beat ia partially separating, not only the molecules, but 



