228 On Evaporation and Dissociation. [Nov. 19, 



IV. " On Evaporation and Dissociation. Part I." By Professor 

 William Ramsay, Ph.D., and Sydney Young, D.Sc, 

 Lecturer and Demonstrator of Chemistry in University 

 College, Bristol. Communicated by Professor Stokes, 

 Sec. R.S. Received August 4, 1885. 



(Abstract.) 



The authors describe experiments made with the object of ascer- 

 taining whether the coincidence of the curves which represent the 

 vapour-pressures of stable solid and liquid substances at different 

 temperatures, with those indicating the maximum temperatures 

 attainable by the same substances at different pressures, when 

 evaporating with a free surface, holds good also for bodies which 

 dissociate in their passage to the gaseous state. The substances 

 examined were chloral hydrate, chloral methyl, and ethyl alcoholates, 

 butyl chloral hydrate, ammonium carbonate, ammonium chloride, 

 aldehyde ammonia, phthalic acid, succinic acid, nitric peroxide, and 

 acetic acid. 



These substances fall into two classes, the first including bodies 

 giving coincident curves, viz., nitric peroxide, ammonium chloride, 

 and acetic acid, the second containing the remaining substances, 

 which do not show such coincidence. Comparing the members of the 

 second class with each other as regards temperatures of volatilisation, 

 it is found that in those cases in which dissociation is complete, or 

 nearly so, the temperatures of volatilisation are independent of pres- 

 sure and do not form a curve. When dissociation is less complete, as 

 with succinic and phthalic acids, a rudimentary curve is observable, 

 and with aldehyde ammonia, which is much more stable, the tempera- 

 tures ca volatilisation form a regular curve resembling a vapour- 

 pressure curve. 



All those substances, however, give curves representing pressures 

 of dissociation, generally similar in form to vapour-pressure curves, 

 and a comparison of these curves with those representing tempera- 

 tures of volatilisation would indicate that the smaller the amount of 

 dissociation the nearer the curves approach each other both in form 

 and position. 



It is noticeable that in the formation of bodies of the second class 

 a molecule of water or ammonia is invariably broken down, whereas 

 with nitric peroxide and acetic acid direct union of like molecules 

 takes place, and there is no such rupture as in the previous cases. 

 As ammonium chloride resembles these substances in the likeness of 

 their behaviour to that of stable solids and liquids, it may perhaps be 



