EXPANSION OF LIQUIDS. 



40o is certainly not very great, being little more 

 than 1 part in 1000 at 32 ; hence it was early 

 suspected that it might be an illusion, from the 

 contraction of the glass bulb (in which the ex- 

 periment was always made) forcing up the water 

 in the stem. But all grounds of objection on 

 this score have been removed by the mode in 

 which the experiment has subsequently been 

 conducted, particularly in the admirable re- 

 searches of Dr. Hope on this subject. 



Dr. Hope carried a deep glass jar, filled with 

 water of the temperature 50, into a very cold 

 room ; and having immersed two small thermo- 

 meters in the water, one near the surface, and 

 the other at the bottom of the jar, watched their 

 indications as the cooling proceeded. The thermometer above 

 indicated a temperature higher by several degrees than the ther- 

 mometer below, till the temperature fell to 40, that is, the 

 chilled water fell as usual to the bottom of the jar, or became 

 denser as it lost heat, as illustrated in Figure 1. At 40 the two 

 thermometers were for some time steady, (Fig. 2.) but as the 



FIG. 1. 



In cooling 

 above 40. 



FIG. 2. 



FEG. 3. 



In cooling 

 below 40. 



cooling proceeded 



beyond that point, 



the instrument in 



the higher situation 



indicated the lower 



temperature (Fig.3); 



or the water now as 



it became colder, 



became lighter, and 



rose to the top. A better demonstration of the fact in question 



could not be devised. 



Great pains have been taken by several philosophers to 

 determine the exact temperature of this turning point, at 

 which water possesses its maximum density. By the recent 

 elaborate experiment of Hallstrom, this point is 39.38. Sir 

 C. Blagden and Mr. Gilpin had made it 39. Dr. Hope had 

 estimated it at 39^. 



When salt is dissolved in water, the temperature of the 

 maximum density becomes lower and lower, in proportion to 

 the quantity of salt in solution, arid sinking below the freezing 



