512 



ILL I hSTRA TI VE EXPERLMENTS 



from those obtained in Expt. (iv.) above 1 Why should a negative pressure 

 be found in some cases ? 



(vi.) Fill the collodion osmometer with distilled water and immerse it in 

 iV/1000 HCl. The water rises in the pressure tube. Why is this so ? 



{h) Determination of the Freezing-point of Urine. Principle. The freezing- 

 point of water is depressed by the addition of salts which go into true solution. 



The magnitude of the depression (termed A) bears 

 a relation to the molecular concentration of the 

 solutes and therefore to their osmotic pressure. 



Apparatus. Beckmann's(Fig. 103). It consists 

 of a specially devised test tube A with a side 

 neck. Through the rubber stopper, closing the 

 main neck of this tube, 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 thermo- 

 meter, if made in the ordinary way, unless it were 

 inconveniently long, w^ould 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 per- 

 mitting of alterations being made in the amount 

 of mercury in the bulb. At the upper end of 

 the thermometer (Fig. 104) there is a small reser- 

 voir 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° higher than the highest temperature 

 to be met with in the experiment and see whether or not the to]i 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, {h) When the mercury passes to the top of the capillary tube and 



Fig. 1U3. — Kreezing-puint 

 Apparatus. 



