HOLMAN. — CALORIMETRY. 253 



is of vital importance to freedom from constant or systematic error, 

 because the duration of the rate r.^ is inevitably greater, often much 

 greater, than of any other rate. If, as in the usual process, therefore, 

 it is a large (often the largest) and the least accurately known rate, 

 it is obviously productive of the most serious part of the error of the 

 coolinf correction. The modified method does not increase the lapse 

 of time preceding the arrival of the final steady condition, D, which 

 must in all methods, directly or indirectly, be the basis from which to 

 deduce t^. And as it does increase the percentage reliability of r,, 

 and reduces its amount, it diminishes the total amount of the cor- 

 rection, thus in a twofold way improving the result. The gain for 

 operations of such a nature that the duration of C B is several or 

 many times that of i? C is large, being approximately in proportion 

 to those dui-ations. Into that category would fall many of the ordi- 

 nary calorimetric processes, all those resembling the measurement of 

 specific heats of liquids which cannot be mixed with that of the calo- 

 rimeter, the use of the bomb for heats of combustion, and almost all 

 processes involving the presence of considerable metallic or glass ap- 

 paratus within the calorimeter. The method is of much less advan- 

 tage where the duration of C D is brief. 



In certain cases there is a slight offsetting increase of error in Vi, 

 namely, when the contents are heterogeneous when both r^ and To are 

 taken. This occurs in the case of the bomb, but not that of the spe- 

 cific heats of liquids or solids. For then r^ is a larger rate, and is in- 

 accurate from the cause under consideration. Inasmuch, however, as 

 this rate has during the operation B C only a relatively short ex- 

 istence, and its error as affecting the value of a is divided by (/o — ^i), 

 the error introduced by it is small. It should be noted that if the calo- 

 rimeter is used as a secondary apparatus, not as a primary or absolute 

 one, the systematic parts of the above errors are further reduced. 

 Thus, for instance, in using the bomb, the heat capacity of bomb, 

 calorimeter, and entire contents may best be ascertained by burning in 

 it a known mass of some pure substance of well determined heat of 

 combustion (e. g. naphthalin), and computing backward from that heat 

 to find the total capacity. This makes the apparatus a secondary one, 

 dependent upon the assumed heat of combustion, but it is much safer 

 than computing the capacity from the assumed specific heats and the 

 masses of the component parts, or than measuring it by the addition 

 of warm water in the well known manner. 



Departure from the third assumption above named cannot be dis- 

 cussed in detail, since the effect varies widely with the change of vol- 



