Biological Chemistry. 



meter must be employed, which is graduated in hundredths 

 of a degree centigrade. The interval of a degree indicated 

 on the thermometer scale must therefore be a long one. 

 By means of this thermometer, which is perhaps the most 

 essential feature of the Beckmann apparatus, the freezing 

 point of a given weight of a pure solvent, and of the same 

 after solution of a known weight of a solvent, can be 

 determined. The difference between the two determina- 

 tions gives the depression of the freezing point. The 

 same form of thermometer is used and similar principles 

 are applied in the determination of the elevation of the 



boiling point. 



Till The Beckmann thermometer is devised not 

 for the measurement of actual temperatures, but 

 rather for the measurement of differences. Al- 

 though the instrument is a long one, the whole 

 range indicated on the scale is not more than 6. 

 In order to use such an instrument over a large 

 range of temperatures, the thermometer is con- 

 structed in a special manner. The mercury is 

 contained in a large bulb which terminates in a 

 very fine capillary tube attached to the scale. 

 At the upper end of the scale, this capillary is 

 widened out into a reservoir (see illustration, 

 Fig. 15). If the bulb of the thermometer is 

 warmed, the mercury can be made to rise through 

 the capillary in a continuous column until it 

 reaches the reservoir. By giving the instrument 

 a sharp tap, the column of mercury in the reservoir 

 can be broken and part of the metal will fall to 

 the bottom and thus be separated from the re- 

 mainder. In this way the amount of mercury in 

 the continuous column from the bulb to capillary can be 

 diminished. It is also possible to increase the amount of 



Fig. 15. 



