POINT OF MAXIMUM DENSITY OF WATER. 



267 



the water at the bottom of the bucket and the other that at the 

 top, will register different temperatures at top and bottom as the 

 water cools. Suppose the water is at 10 to start with ; as the 

 cooling goes on the top thermometer will at first indicate a higher 

 temperature than the bottom one, because the cooler water is 

 heavier and tends to sink to the bottom ; by and by the two 

 thermometers will indicate the same temperature, 4 ; after which 

 the top thermometer will register a lower temperature than the 

 other, because now the cooler water is higher and rises ; finally, 

 ice will form on the top. Thus, if readings of the two ther- 

 mometers be taken every 5 minutes, numbers will be obtained 

 something like the following: 



Fig. 122 represents a form of arrangement for trying this ex- 

 periment in an ordinary room ; a large cylinder containing water 

 has a sort of gallery arranged half way up, filled with fragments 

 of ice, so as gradually to chill the water, when numbers will be 

 obtained on taking readings of the two thermometers somewhat as 

 above. It is probable that all substances that expand in solidifying, 

 like water, exhibit the phenomenon of "maximum density;" but 

 in most cases it is very difficult to prove that this is so, as the 

 point of maximum density lies but little above the melting point. 



Expt. 316. Expansion of Gases. Fill a bladder nearly, but 

 not quite, full of cold air with a pair of bellows, and then keep 

 it in a warm place, e.g., in front of the fire, but not so near as to 

 burn and shrivel the membrane ; the bladder will soon become 

 plump and fully distended, and if not very strong may even burst 

 from the increased pressure, due to the tendency of the air inside 

 to expand by heat. 



Fig. 123 represents an arrangement illustrating in another way 

 the development of pressure by this action. A glass bulb with a 

 long cylindrical stem (a piece of wide tubing attached to a flask 



