CH. I.] COLLOIDS. 9 



The tube B serves as an air jacket to A. Stir the water regularly 

 by means of the (platinum) stirrer E. The temperature falls, and 

 then after a time rises sharply, and remains steady for a considerable 

 time. The temperature to be read is the highest obtained at this 

 rise. This is the freezing point (W) of distilled water. 



Now replace the water by the fluid, rinsing the tube out with 

 it once or twice. Repeat the experiment and note the freezing 

 point (F) as before. W F = A. 



NOTES. i. It is of the utmost importance to take care to prevent too 

 great a super-cooling of the fluid. This should never exceed i C. If it has 

 exceeded this in a preliminary experiment, it must be repeated, and when the 

 temperature has fallen 0-5 C. below the freezing point, a minute crystal of ice 

 must be introduced through the side tube. These crystals are best prepared 

 by taking, in a dry test-tube, some hollow glass beads (that have been care- 

 fully dried), adding a small amount of the fluid, pouring off the excess, and 

 immersing the tube in a freezing mixture. They should be introduced by 

 means of a pair of cooled forceps. 



2 The observed A is usually too great, owing to the super-cooling. The 

 simplest correction is 



A corrected = A observed x I i - ^- 1 



where C = the super-cooling in degrees. 



3. To set the thermometer. Turn the thermometer upside down, and by 

 gentle shaking mix the mercury in the upper portion with that in the capillary 

 tube. Then place the thermometer in water at about 2 C. Give a slight 

 knock, and thus break the mercury column. It is now ready for use. 



4. When reading the thermometer during an experiment it should be 

 tapped with a piece of indiarubber tubing. 



E. The electrical properties of colloids. 



Under certain conditions it is found that colloidal 

 particles carry an electric charge. In some cases they 

 exhibit electrical conductivity, due to the fact that the 

 substances are partially ionised. But even if they do not 

 exhibit this phenomenon it is often found that they tend to 

 move towards one of the poles when a strong ( 100 volts) con- 

 stant current is sent through the solution. In some cases 

 this movement (" kataphoresis ") is towards the anode, i.e.* 

 the particles carry a negative charge ; in other cases it is 

 towards the kathode. It is important to note that the 

 direction of the migration can be changed in many cases by 

 varying the reaction of the fluid in which the colloid is 

 suspended. Thus metaproteins, albumins, etc., carry a 



