RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 515 



which, though heated, is not quite as high in temperature as the boil- 

 ing point, and consequently premature condensation is not wholly 

 avoided. By no means all of the heat thus lost finds its way into the 

 calorimeter. The result of this defect is to give a value lower than the 

 true heat of vaporization. 



In the hands of Louguinine ^"^ the apparatus of SchifF has been so 

 perfected as to make it one of the most satisfactory heretofore used. 

 The tube leading to the trap was made large in order to prevent the 

 clogging with condensed liquid. The proximity of the hot trap to the 

 calorimeter necessitated a correction for heat gained therefrom through 

 radiation and conduction. In Schiff's work this correction appears to 

 have been omitted, but Louguinine evaluated this cause of error by 

 keeping the trap hot while he prevented both vapor and liquid from 

 entering the condenser. In this way the heat gained per minute by 

 radiation and conduction can be determined fairly well, and, knowing 

 the time during which vapor enters the condenser and gives up heat, a 

 suitable correction can be made for the heat gained by radiation and 

 conduction during the same period. Louguinine made the distance 

 from the trap to the condenser very short (15 mm.) in order to 

 reduce the premature condensation of the vapor, but our experience in- 

 dicates that even then he could not have been wholly successful. The 

 fact that his results for the heat of vaporization of water are fairly 

 concordant does not prove the absence of error from this cause. To 

 reduce condensation to zero, the distance between trap and condenser 

 should also be reduced to zero, — an obvious impossibility. Because, 

 as has been said, the effect of the error is to make the heat of vapori- 

 zation appear less than it really is ; Louguinine's three values, 535.61, 

 537.61, and 538.51 calories per gram are probably too low. 



About fourteen years ago J. A. Harker^^ published an elaborate 

 and interesting account of his work. His early rejected experiments 

 demonstrated more conclusively than ever the danger of premature 

 condensation, and he concluded that this cause of error is inevitable, 

 when the vapor is introduced from above. In his later work, fearing 

 that minute drops of water exist in vapor as ordinarily formed in dis- 

 tillation, he passed the vapor through coils in a heated oven, and then 

 through a zone kept at a constant temperature about three degrees 

 above the boiling point. Finally the vapor was admitted into the con- 

 densing coil of the calorimeter, through the side of the latter, the 

 temperature being taken at the point of introduction by means of 



" Ann. Chim. Phys., (7) 7, 251 (1896). 



18 Mem. Manchester I^it. and Phil. Soc, (4) 10, 38 (1896). 



