METHOD OF DETERMTXIXG MOLECULAR WEIGHTS. 



33 



the criterion f<^r ja<lging that the boiler and tlie boiling vessel are 

 each being heated by a properly adjusted lamp, so that evaporation and 

 condensation balance each other in the boiling vessel. The thermo- 

 meter rises raj)idly at first but more slowly afterward and attains the 

 maximum temperature in 10 or 12 minutes, which then remains per- 

 fectly constant. The top of the tliermométer is now lightly tapped for 

 some seconds, and tlie observation of the exact temperature made by 

 tlie aid of a reading telescope. Finally, the volume of the distillai e 

 collected in the sm:dl flask F is noted and, after Avithdrawing the 

 lamps, the stoji-cock r is opened and the stop-cock s closed. 



I make it a rule to repeat this observation of the exact boiling 

 ])f)int of the solvent once or, in some cases, twice more in order to 

 make quite sure of the temperature. For this purpose, the distill-ite 

 is returrjed to the boiler arid the whole of the al)Ove process repeated 

 under exactly the same conditions as before, taking the final observa- 

 tion of the temperature "when the distillnte reaches the n^ark previously 

 made (^n the receiver flask. 



The next ste]) is to introduce the suljstance into the solvent. 

 This may l)e d(^ne in two ways. The first and simplest method, 

 which should be used when the solvent is of low boiling point, con- 

 sists in taking out the cork, which carries the thermometer, adding 

 the solid substance to the solvent, and then immediately replacing 

 the cork to it original position. The other method, which is employed 

 in the case of solvents of high boiling point and where, therefore, if 

 the first method were adopted, the highly heated thermometer would 

 have to he suddenly exposed to the cold air, is as follows : After 

 the exact determination of the boiling point of the solvent has been 

 mnde, the boiler is ;i little cooled witiiout opening the stop-cock r, 

 when a small (|u:intity (n"" the solvent runs up the tube e and fiows 

 down into the Ijoilei-. The stop-cock r is now opened and then, dis- 



