RICHARDS AND MATHEWS. — HEAT OF EVAPORATION. 527 



0.093. It may be noted that the weight of the copper vessel need not 

 be known within two or three grams, for this corresponds to the limit 

 of possible accuracy of the thermometric part of the experimentation. 

 The pure tin condenser weighed 486.7 grams, and therefore had a heat 

 capacity equivalent to 23.6 grams of water, the specific heat of tin at 

 21° being about 0.054.^7 The thermometer was found by the method 

 of Ostwald-Luther 28 to have a heat capacity equivalent to 1.4 grams of 

 water ; and the copper stirrer, weighing 7.7 grams, had a heat capacity 

 of 0.7 on the same basis. These weights have as their sum 53.4 grams. 



An important matter of detail lay in the discovery of the time 

 needed for equalization of temperature between the hot liquid accumu- 

 lating in the condenser and the water of the calorimeter. In order to 

 test this, a glass funnel was substituted for the vaporizer at B, and 17 

 grams of water were poured little by little through this funnel during 

 5 minutes. There being no heated object near the calorimeter, the 

 radiation-effect was negligible, and the rise in temperature of the cal- 

 orimeter water was due only to the hot water introduced. 



Immediately after the addition, the reading of the thermometer was 

 a trifle over 1° ; in another minute the thermometer read 1.101° ; in 

 yet another minute 1.104°. In the 3 minutes following, the thermom- 

 eter rose 0.001° each minute, and finally remained perfectly constant 

 at 1.107°. Thus in 5 minutes after the last portion of water had been 

 added a constant temperature had been attained, showing that with 

 the rate of stirring usually adopted, this time was sufiicient for com- 

 plete equalization of the temperature within the calorimeter. 



The calorimeter was surrounded by a narrow air-space, bounded by 

 a copper can (E) with a burnished nickel-plated lining. This was im- 

 mersed in a much larger vessel (F) of about ten liters capacity, which 

 contained dilute crude sodium hydroxide. The outer vessel was 

 provided with a basin-shaped cover (G) of about four liters capacity, 

 through which were several openings for thermometers, vaporizer (V), 

 stirrer, etc. The bottom of the cover was coated with bright tin-foil. 

 In this way the calorimeter was entirely surrounded by a uniform tem- 

 perature, except where the vaporizer protruded through the cover. A 

 powerful stirrer (H), revolving at the rate of 250 revolutions per minute 

 and driven by an electric motor, kept the lower alkaline solution in 

 violent agitation, while the solution in the cover was more gently agi- 



*'' The specific heats of copper and tin usually given (0.094 and 0.055 re- 

 epectively) correspond to the range between 20° and 100°. The values given 

 above take account of the decrease with decreasing temperature. 



28 Ostwald-Luther, Handbuch, p. 300 (1910). 



