Physical Properties of Water 271 



Some experiments will next be described in illustration 

 of the three points just mentioned. 



EXPERIMENT 123. Into a small beaker, half filled 

 with distilled water, drop a little powdered alum. 

 Stir the water and alum well with a glass rod. Should 

 all the alum be caused to dissolve by this method, add 

 a little more, and keep the mixture well stirred for 

 several minutes after no more of the alum seems to 

 disappear. Drop six drops of the solution thus obtained 

 on a watch-glass and evaporate to dryness. Put the 

 beaker on a piece of wire gauze on a tripod stand and 

 heat it with a good Bunsen flame. Drop in small 

 quantities of alum from time to time until no more 

 will dissolve, even though the liquid be boiled vigorously 

 for a full minute. Evaporate six drops of this solution 

 as before. From the comparison of the amounts of 

 residue obtained in the two cases, and by noting 

 roughly the amounts of alum dropped into the cold and 

 the hot water respectively, it will be seen that alum 

 is very much more soluble in hot water than in cold. 

 Allow the hot solution to cool. As it cools, small 

 , particles of alum begin to appear in the liquid, chiefly 

 at the bottom of the beaker. By the time the solution 

 has reached the temperature of the surrounding air, 

 most of that previously dissolved will have thus 

 separated out. Examination of the alum at the bottom 

 of the vessel may show that the particles are of regular 

 shape, but usually they are too small to show this, 

 when examined by the naked eye. To obtain larger 

 particles the solution should be cooled very slowly 

 or a single small particle should be immersed at the end 

 of a thread attached to a glass rod, the latter resting 

 on the mouth of a beaker containing the remainder 

 of the solution poured off from the solid which has been 



