LABORATORY WORK ' 177 



As another example of a suspensoid, skake up kaolin (china 

 clay) with distilled water, and allow the coarser particles to settle. 

 The finest particles will remain dispersed for some hours, and such 

 a preparation serves well for experiments. 



Emulsoids. Prepare a solution of gelatin by allowing a sheet of 

 the dry substance, as sold, to soak in water until softened, and 

 then dissolving it in hot water. As the solution cools, it sets to the 

 familiar jelly. 



Another emulsoid which does not set to a jelly is white of egg. 

 This has another property, that of becoming solid when heated, as 

 well known. In this state its properties change to those of a 

 suspensoid. 



The increased swelling of gelatin in the presence of acid may 

 be shown thus : Allow a sheet to soak in water. Cut out a number 

 of discs with a cork borer of about c cm. in diameter. Place some 

 in distilled water, others in deci-normal hydrochloric acid. After 

 some hours measure the diameters of a few of each. 



Surface Tension and Dispersion. Soap has a powerful effect in 

 lowering surface tension. Olive oil usually contains a small 

 quantity of free oleic acid, and when alkali is added, this forms 

 soap (see later, page 190). Olive oil shaken with water forms an 

 emulsion, but the drops of oil quickly coalesce and rise to the 

 surface. If a very small amount of sodium hydroxide be added 

 and the mixture again shaken, the soap formed lowers the surface 

 tension at the contact of the water and oil, so that the drops 

 have little tendency to unite, and a nearly permanent emulsion is 

 produced. 



Electrical Charge. The simplest method of determining the 

 sign of the charge on colloids is to take a U-tube of about a centi- 

 metre in diameter. Fill with the solution, and place in it at the 

 upper end of each limb a piece of platinum foil connected with a 

 source of potential difference of some 200 volts, such as the direct 

 current house lighting supply. One electrode will be positively 

 charged ; the other negatively. A lamp should be inserted in the 

 circuit to diminish risk, should the electrodes be accidentally brought 

 into contact. The sign of the poles is determined by placing the 

 two electrodes 2 or 3 cm. apart on a piece of filter paper wetted 

 with a solution of sodium sulphate to which phenol-phthalein has 

 been added ("pole-finding paper"). The negative pole produces a 

 red stain, owing to the alkali formed. After the connection to the 

 tube has been made for some minutes, the space around and below 

 one of the poles will become clear, owing to the repulsion by the 

 electrode of particles of the same sign as itself. 



Two typical suspensoid colloids are arsenious sulphide and 

 ferric hydroxide. The former is negative ; the latter, positive. 



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