Reaction before Complete Equilibrium. 273 



(which gives the value of the experimental error). The difference 

 between the real and apparent lowering of the vapour-pressure 

 or of the rise of boiling-points, fyc, is 



and the value of the right side of the equation must be kept 

 by the arrangements of the experiment as small as possible, 

 so that their value may be neglected on the side of t —tj. 



The present considerations are only of a preliminary nature, 

 and it is hoped in the future to give a i'ull statement of the 

 subject. 



The boiling-point method gives moderately satisfactory 

 results if very great accuracy is not required. The velocity 

 constant of evaporation C" is moderately great at the boiling- 

 point, the equilibrium being always reached in a comparatively 

 short time. So also 100 c. cm. of liquid and an air-bath are 

 the quantities commonly used (Beckmann). Thus C ;/ is 

 great enough and C is not very small. Further improvements 

 on the method will be made by the use of a greater quantity 

 of liquid, and by the arrangement of the convergence tem- 

 perature as near as possible to the boiling-point of the given 

 liquid. 



With the methods for vapour-pressures we have also been 

 unable up to now to get satisfactory results, even in the case 

 of moderately dilute solutions, where the boiling-point method 

 is already capable of giving satisfactory results. The reason 

 of this is to be found in the fact that the velocity constant 

 C" of evaporation is very small. As is the case with all 

 reactions, the velocity of evaporation becomes smaller with 

 the fall of temperature ; and it is a well-known fact that the 

 time necessary for obtaining the maximum vapour-pressure 

 is, at ordinary temperature, already very considerable. Since 

 the total value of the observed vapour-pressure is at the 



ordinary temperature very small, the values of * — - and 



ot -77 — fa — '- torm a considerable part of the value 



of T'-T" or of T - TV. It was shown (Phil. Mag. Dec. 1897) 

 that even in the freezing-point method, when 100 c. cm. of 

 liquid, a liquid-bath of — o, 3, and a quantity of ice at equili- 

 brium = 0°*o are used, the experimental error in the obtained 

 freezing-points and freezing-point depressions becomes already 

 about 200 times greater than in my method, and amounts to 

 several thousandths of a degree. It is clear that the results 

 Phil. Mty. S. 6. Vol. 4. No. 20. Aug. J 902. T 



