QUANTITY OF HEAT. SPECIFIC HEAT. 



67 



The vessel or calorimeter containing the water (Fig. 52) should be 

 of thin metal polished on the outside thin that it should not absorb 

 much heat, polished that it should lose little by radiation. It should 

 have a lid, and should be supported by badly conducting material, 

 with as small surfaces of contact as possible, say pointed wood or ebonite 

 pegs, and should be surrounded by an outer vessel, preferably a water- 

 bath, kept at a constant temperature. An intermediate thin metal 

 vessel, highly polished on both sides, may still further diminish the 

 radiation loss. To allow for the quantity of heat absorbed by the 

 calorimeter, thermometer, and stirrer, we must find their capacity or 

 water equivalent, and regard it as so many grammes of water in 

 addition to that actually contained in the calorimeter. This water 

 equivalent is determined by putting into the calorimeter about the same 

 quantity of water as "will be used in the final experiment with the brass, 

 and by adding some hot water so as to 

 produce about the same rise of temperature 

 as is expected in that experiment, and 

 noting the exact rise produced. 



Thus, suppose that the calorimeter con- 

 tains 100 grammes of water at 15,* and 

 that 40 grammes of water at 48 are poured 

 in. The contents are well stirred, and in 

 half-a-minute the temperature has risen to 

 23'55. Meanwhile, however, the calori- 

 meter has been losing heat. To estimate 

 this loss the rate of fall is now observed. 

 Suppose that one minute after the last 

 observation the temperature is 23 '35, and 

 one minute later still 23 -15. The rate of 

 loss corresponds to a fall of 0'2 per minute, 

 or of O'l per half -minute. But during the 

 rise of temperature the rate of loss may 

 be taken as half this on the average ; for 

 supposing the temperature to rise uniformly 

 from the moment of mixture to the 

 maximum observed only an approximation to the truth, no doubt the 

 average excess above the surrounding enclosure is only half the final 

 excess, and the loss, which, for small excesses, may be taken as pro- 

 portional to the excess above the surroundings, is at only half the rate 

 of the final loss observed. Thus, had all the heat been kept in, the 

 temperature would have been x 0*1 = '05 higher, or the corrected 

 temperature is 23 6. f 



Expressing that the gain of heat by calorimeter and contents = loss 

 by hot water, and putting w for the water equivalent to be found 

 (w + 100) (23-6 - 15) = 40(48 - 23-6) 



* In mere demonstration experiments it is easier to work with small quantities, 

 but, if exact results are required, then larger quantities, say not less than 500 

 grammes of water, should be used. Otherwise the corrections are too large a 

 fraction of the whole effect. 



t In accurate work much more care must be taken with the correction for 

 loss of heat. A description of the mode of doing this will be found in Ostwald's 

 Physico-Chemical Measurements, p. 126. 



