116 0. Barns — Compressibility of Hot Water, etc. 



aware that the march of t Q is to be interpreted with reference 

 to Wilhelmy's* time law of reaction ; but the discussion is 

 somewhat involved and must be omitted. 



Curiously enough the effect of solution is here an increase 

 compressibility, whereas in all other cases (§ 2), it is a decre- 

 ment of compressibility. I leave this without comment, 

 believing however, that the silicate during the course of the 

 oscillations of pressure, passes through states of unstable 

 equilibrium with its water. The apparent compressibilities 

 measured, are really solution phenomena, since the silicate 

 present passes from a lower to a higher state of hydration 

 when pressure passes from the lower to the higher value. For 

 this reason compressibility increases with the quantity of 

 silicate present, in other words with the time during which the 

 solvent action has been going on. Something of this kind I 

 formerly observed in case of moist mono-chloracetic acid. The 

 possible occurrence of lag, though I did not search for it, 

 would be obscured by the contraction of the silicated column 

 of water. If a reaction is superinduced' in a system of solid 

 and liquid by presssure, the curious question is thus presented 

 whether the reaction is more complete in proportion as the 

 acting pressure is higher ; or more generally whether the final 

 progress of the reaction, or the chemical equilibrium varies 

 with pressure. To my knowledge in all the relevant instances 

 examined (take the action of acid on zinc under pressure of 

 the gas evolved) this is true. 



As a general deduction from the above experiments I infer, 

 that in many instances a definite dissociation temperature of 

 the solid must first be surpassed, before solution will set in. 

 Elsewhere I shall show that the recognition of this principle, 

 regarded at the outset as a mere working hypothesis, has 

 enabled me to effect the complete solution of a valuable class 

 of commercial products. 



* Wilhelmy: Pogg. Ann., lxxxi, pp. 413, 499 ? 1850. 



