HOLMAN. — CALORIMETRY. 249 



At to allow for unavoidable rise during preliminary manipulations, — 

 in short, so that ti° shall be as nearly as may be A t° lower than 6°. 

 The object of this is to have <2 so nearly that t-j shall be nearly 

 zero. On the plot of observations, Figure 1, this would cause ^ ^ to 

 incline upwards and D E to become nearly horizontal. 



Then proceed precisely as in the general method, both as to manipu- 

 lation and computation. 



Theory of the Methods. 



A typical set of readings obtained under the general method, cor- 

 rected for tliermometric error, and plotted with times as abscissas and 

 temperatures as ordinates, would lie along a line A B C D E, Figure 1. 

 This curve should be convex upward all the way from C to D, there 

 becoming straight. If any concavity shows itself between the maxi- 

 mum M and E, or if the straight line through the points between D 

 and E cuts the curve anywhere between C and D, then the data 

 should be rejected. For the inference, almost without exception, is 

 that the stirring was insufficient to keep the thermometer down to the 

 average calorimetric temperature. This may be due either to the 

 bulb being badly located, so that it is unavoidably heated to excess, or 

 merely to ineffective stirring. The difficulty should be removed, and 

 the observation repeated. 



The line consists of four portions, viz. : 1. The preliminary read- 

 ings, A to B. 2. The readings during the operation, B to ?ome inde- 

 terminate point C. 3. Readings G to D while the calorimeter and 

 contents are becoming equalized in temperature. 4. Final readings 

 after this equalization has become sensibly completed, and the calo- 

 rimeter and contents are cooling, or heating, by exchange with their 

 surroundings. In many cases, as in specific heat determinations, the 

 second and third parts are one ; that is, the operation is the equaliza- 

 tion of temperature between the calorimeter and the substance intro- 

 duced into it. In the case of a combustion and many other operations 

 these two parts are to be distinguished. 



The method is based upon Newton's law of cooling. Test observa- 

 tions have shown that this law holds with sufficient accuracy for both 

 open and closed calorimeters, that is, within the uncertainty attribu- 

 table to varying conditions of surroundings and to thermometric 

 errors. 



From the best representative straight line drawn through the ob- 

 served points between A and 7? is deduced the initial rate of exchange, 



