400 



ILLUSTRATIVE EXPERIMENTS 



D 



Apparatus. Beckmann's (Fig. 74). It consists of a specially devised 

 test tube A with a side neck. Through the rubber stopper, closing the 

 main neck of this, pass a thermometer D and a short glass tubular 



guide for a stirrer. The freezing point tube 

 is supported in the neck of a large test tube 

 B, by means of a cork or asbestos ring so 

 that the freezing-point tube is protected from 

 incoming heat by a mantle of still air. This 

 ensures that the cooling of the liquid in the 

 freezing-point tube is slow and fairly uniform. 

 The whole apparatus is inserted through a hole 

 in the middle of a brass sheet, to which it is 

 fixed by a ring of cork or of asbestos. The 

 sheet of brass acts as a lid to a glass jar C which 

 contains powdered ice and salt the cooling 

 bath. Other holes in the lid permit of the 

 passage of a stirrer, a thermometer, and a test 

 tube containing pure water. 



The Beckmann Thermometer. The thermo- 

 meter in the freezing point tube must be 

 graduated to, at least, hundredths of a degree. 

 Such a thermometer, if made in the ordinary 

 way, unless it were inconveniently long, would 

 have a very short range. To obviate the 

 necessity of having a series of thermometers 

 for use over various ranges of temperature, 

 Beckmann designed one which may be set to 

 indicate temperatures over any desired range. 

 This result is produced by a device permitting 

 of alterations being made in the amount of 

 mercury in the bulb. At the upper end of the 

 thermometer there is a small reservoir into 

 which the excess of mercury may be driven, or 

 from which a larger supply of mercury may be 

 obtained. 



Setting the Beckmann Thermometer. Hang 

 the thermometer in a beaker of water, the 

 temperature of which is 2-3 degrees higher than 

 the highest temperature to be met with in the 

 experiment and see whether or not the top of 

 the mercury comes within the scale. 

 A. If there is too much mercury in the bulb and the column rises beyond 

 the graduated part, the excess is removed by warming the mercury in the 

 bulb till the column of mercury unites with the mercury in the reservoir. 

 This is done, (a) by placing the bulb in water just a little warmer than 

 before, (b) When the mercury passes to the top of the capillary tube and 

 forms a small drop there, the thermometer should be carefully inverted 

 and tapped gently so as to cause the mercury in the reservoir to coalesce 

 with the mercury in the top of the capillary, (c) The thermometer is 

 returned to the upright position by a gentle steady movement and its 

 upper end is struck a sharp tap against the palm of the hand, causing 



FlO. 74. Freezing Point 

 Apparatus. 



