274 Dr. Meyer Wilderman on the Velocity of 



obtained with the different methods of measuring vapour-pres- 

 sures must be considerably worse than those of the freezing- 

 points. In the first instance the equilibrium in the case of 

 freezing-points is completely reached after about 15 seconds, 

 while in thecaseof vapour-pressures it takes hours: therefore the 

 value of G" is in the case of vapour-pressures several hundred 

 times smaller. In the second instance the quantity of the liquid 

 used to the present in these methods is, in comparison with that 

 now used for freezing-points, very small, a liquid jacket is 

 always employed, and the surface of evaporation T«>— T w cannot 

 be increased ad libitum, as in the case of freezing-points. No 

 care we find is also taken for getting the convergence-tem- 

 perature near the point of equilibrium. To this we must add 

 other very serious sources of error, such as the variation of 

 concentration and temperature at the evaporating surface, 

 which cannot be easily avoided, &c. From the above remarks 

 it is clear in what direction, on which lines, and how far the 

 methods for measuring vapour-pressures can be effectually 

 advanced. We cannot change the value of C x/ , but we can 

 use greater quantities of liquid, try to keep the evaporating 

 surface larger, keeping the liquid rather wide than deep; we can 

 provide the who'e apparatus with an air-bath placed in a 

 liquid- or vapour-bath of a constant temperature, &c, and get 

 the convergence-temperature as near as possible to the point 

 of equilibrium (there is almost an unlimited number of liquids 

 or solutions to choose from, boiling at any desired tempera- 

 ture), so also we have to provide some arrangements for 

 shaking the liquid. It is obvious that the values of C H and 

 of T — T or make it in this case very imperative that great 

 effort should be made to get the value of T^— T very small. 

 The experiments will, because of this, necessarily be very 

 tedious, but it is unavoidable. No doubt the results will always 

 remain less accurate than may be desirable, limited as we are 

 by the nature of the phenomenon itself in our capacity of 

 making progress. It is further to be considered that an 

 error of 0*001 mm. mercury causes for one molecule normal 

 solution of a nonelectrolyte already an error of 1*2 per cent, 

 in the total value of the observed depression of the vaponr- 

 pressure, and 12 per cent, for 0*1 mol. normal solution, while 

 in the freezing-point methods even an error of 0°*0015 does 

 not form 0*1 per cent, error in the total observed value of the 

 freezing-point depression of one molecule normal solution 

 (the aggregate error of ray method, however, does not reach 

 the value of 0°*0(J01-0 o *0003). For this reason also much 

 more accurate readings than to 0*001 mm. mercury become 





