of Mixing- Calorimeter. 251 



[150-82— (-23x1-5) = ] 150'48 millim. But, according to 

 the previous comparison of the thermometers, the reading of 

 '08 which corresponds to 144*36 of '61 is 150'28 millim. ; 

 hence the initial temperatures corrected for this difference 

 will be, according to '08 (150*48 — -10) millim., and according 

 to '61, (144*36+ (-10 xiJL 3 )) millim. ; the fraction l££ repre- 

 senting the relative values of 1 millim. of the scale of the two 

 instruments. 



The liquids had entirely mixed in this experiment at W 0", 

 and the readings of the two instruments at this moment would 

 be [197*99 (mean of the last four readings) -I- (-62 X 2) =] 199*23 

 millim. in the case of '61, and [194*76+ (-48 x 2'5) = ]195'96 

 millim. in the case of '08. During the one minute while the 

 liquids were mixing, the rate of cooling is taken to be the 

 mean between the rates at the initial and final temperatures, 

 necessitating the further addition of -44 and -36 millim. to the 

 readings last mentioned. The only other corrections to be 

 then applied are those for the effect of the temperature of the 

 surrounding air on the mercurial columns, and the calibration 

 corrections. 



Where one liquid is very much more dense than the other, 

 as in mixing water with salt-solutions of a density 1*3 to 1*7, 

 two special difficulties arise : the temperature of the salt- 

 solution rapidly rises to an appreciable extent above that of 

 the water, owing to the greater friction of the stirrer in the 

 former ; and, secondly, the two liquids mix very slowly when 

 the doors are opened, taking, in extreme cases, nearly 30 

 minutes to do so. This latter is not, however, entirely due to 

 the viscous nature of the liquid, but to the fact that the two 

 stirrers in my present apparatus rotate in opposite directions, 

 whereas for the purpose of rapidly mixing the liquids they 

 should rotate in the same direction. 



These objections are, however, of no great importance. 

 They do not exist in cases where there is a difference of -2 or 

 less in the densities of the solutions, and in very few cases 

 would the difference reach this amount. 



The exactitude of the method may be estimated by the fact 

 that the difference in the rise or fall of temperature as mea- 

 sured by the two thermometers in all the experiments which I 

 have yet performed with the apparatus (omitting those in which 

 the liquids were too dense to mix in less than three or four 

 minutes) is, on the average, only o, 0008, and only on three 

 occasions did it exceed twice this amount. This quantity 

 represents a sum total of *07 millim. error in all the readings 

 of the two thermometers, and represents a difference of 0*6 

 cal. in the heat measured, taking the actual quantities used 



U2 



