1 72 INTRODUCTION TO GENERAL PHYSIOLOGY 



be first well soaked in warm gelatin solution, the edge of the glass 

 being also immersed in the solution. After removal of both, the 

 paper is left in position on the glass until the gelatin is set. It 

 should then be hardened by immersion in 5 per cent, formalin. It 

 is necessary to support it by a disc of nickel gauze, tied on by 

 string or wire brought over the top of the glass. A narrow U-tube 

 containing mercury is fixed in a rubber cork in the upper opening 

 of the glass. 



The caseinogen solution is made by taking the required amount, 

 rubbing it in a mortar with a little water to which a drop of phenol- 

 phthalein has been added. Ammonia is dropped in until the colour 

 has become pink. The volume is then made up in a measuring 

 flask to the required value. 



If possible, an accurate measurement should be made of the 

 osmotic pressure of the colloids of blood serum, using as the outer 

 liquid a 0.9 per cent, sodium chloride. 



Exact determinations of freezing and boiling points are also 

 rather difficult. The student should take that of distilled water by 

 immersing a test-tube, containing a few c.c. of it, in a mixture 

 of ice and salt ; there should not be a large proportion of salt. A 

 thermometer is immersed in the water, which must cover the bulb. 

 Continually stirring by means of a ring of wire fixed in a glass 

 tube by means of sealing-wax the temperature will fall steadily to 

 o, and remain so for a little time. Taking out the tube from the 

 freezing mixture, particles of ice will be seen floating in it, or a 

 thin layer of ice may be present on the inside of the tube. The 

 temperature will be seen to remain at zero until this ice has melted, 

 provided that the stirring is continued. Repeat with a molar 

 solution of sodium chloride (5.85 per cent). The temperature will 

 fall lower before remaining steady, and will remain at this Jower 

 temperature on removal until the ice has disappeared. Only a 

 small amount of ice should be allowed to form, because it is pure 

 ice (frozen water) that separates, leaving a more concentrated salt 

 solution. The lowering of the freezing point by molar sodium 

 chloride is about 3. 



In connection with the problem of electrolytic dissociation, it 

 is well at this stage to compare the freezing points of molar solu- 

 tions of sodium chloride and of urea. We have already found that 

 of the former. Make a molar solution of pure urea ( = 6 per cent). 

 It will be found to have a freezing point of 1.86 only. 



A rough measurement of the boiling point of a solution may 

 be made by the use of a flask in which it is kept boiling gently, 

 while the bulb of the thermometer is suspended just above the 

 level of the liquid. A bit of porous clay, such as the stem of a clay 

 tobacco pipe, put in the liquid, will assist steady gentle boiling. 



