38 



HEAT. 



indicated in Fig. 27. A freezing mixture is put into the gallery. At 

 first, the water is some few degrees above the point of maximum 

 density. The water in contact with the side near the gallery becomes 

 cooled, and, therefore, more dense, and sinks, while the warmer, lighter 



water from below rises up 

 and takes its place, and 

 circulation of the liquid is 

 produced. This readjust- 

 ment of unequally heated 

 matter by its change of 

 density is an instance of 

 " convection." There is, at 

 first, no circulation in the 

 upper half of the vessel, 

 and the higher thermometer 

 only cooled by conduc- 



is 



Time 



FlG. 28. Indications of Thermometers in 

 Hope's Apparatus. 



tion, which takes place but 

 slowly. Its fall is, therefore, very gradual. But the denser water falls 

 to the bottom in the lower half, and the lower thermometer falls rapidly. 

 The circulation goes on until all the lower part has become of the 

 maximum density. Then the circulation ceases, for further cooling only 

 makes the water lighter, and it does not descend. The cooling of the 

 lower layers can now only take place by con- 

 duction, and is very slow. When uniformity 

 is reached in the lower half, the further cool- 

 ing of the middle strata leads to circulation 

 upwards, and this will go on to a higher and 

 higher level until the upper thermometer is 

 reached, when its fall will be much more rapid. 

 The upper part may now fall below the point 

 of maximum density, for the lighter colder 

 water goes on rising upwards. 



Conduction may now begin to affect the 

 lower thermometer, and it may fall somewhat 

 more rapidly. Fig. 28 represents a series of 

 observations on a time-temperature diagram 

 the long, nearly horizontal portion of the 

 lower thermometer indicating a temperature 

 in the vicinity of that of the maximum 

 density. 



Apparatus of Joule and Play fair. Pro- 

 bably the most accurate mode of determining 

 the point of maximum density is that of 

 Joule and Playfair,* based on a principle FlG< 2 9. Apparatus of Joule 

 similar to that applied in Hope's apparatus. and Playfair for Point of 

 Two tall iron cylinders A and B (Fig. 29) Maximum Density of Water. 

 are open above, and connected by a trough 



T of about one inch cross-section. In this is placed a glass bulb only 

 just floating. Below is a cross pipe P, furnished with a stop-cock. 



The cylinders are filled with water, one just above, the other just 

 * Joule's Scientific Papers, vol. ii. p. 173. 



B 



