270 



Methods of determining and calculating 



only must AH = the constant G, but also A(H+(ZH) must = C', 

 and of course AcZH = C, — in order that another body having the 

 atomic weight a, and a specific heat A, should give ah = C, a[h-{-dh) 

 —C', and adh=C". Let us observe how far their table of the in- 

 crease of specific heat between 212° and 572° Fahr. will bear out 

 this supposition. The atomic weights are those given by Petit and 

 Dulong themselves, with the exception of those of mercury and an- 

 timony, which are derived from Dr. Thomson. 



Petit and Dulong's Values of C" 



difference of specific heats ^ ^^^^^ ^^j^_ 

 between 212° and 572° F. 



.0020 

 .0064 

 .0020 

 .0054 

 .0064 

 .0120 

 .0088 



.025000 

 .023100 

 .022520 

 .036450 

 .025280 

 .040704 

 .035464 



To attribute the character of " constants'^ to such numbers as are 

 found in the fourth column of this table, would be little satisfactory 

 to any who were not prone to uphold a theory at all hazards. Even 

 the apparent correspondencies between mercury and copper, anti- 

 mony and platina, silver and zinc, are probably mere accidental co- 

 incidences. Iron, on which the authors to whom I have referred, 

 appear to have bestowed most attention, gives a result far removed 

 from all the rest and nearly double to some of them. 



The foregoing considerations, together with the use to be made 

 of the specific heat of iron and platina in generating vapor for pyro- 

 metrical measurements have induced me to attempt a re-examination 

 of certain parts of this subject, and for this purpose I have taken the 

 method originally adopted by Wilcke and Black, viz. that of im- 

 mersing the hot metal in cold water, in connection with the fourth 

 method above described, that of using the latent heat of vapor to as- 

 certain the specific heat when the temperature of the solid is known. 



In experiments of this nature several precautions are to be ob- 

 served, and a considerable number of sources of error anticipated, 

 against which, if we cannot directly guard, we must provide for 

 them the necessary correcti.ons. 



1 . We must attend to the character and condition of the metal, 

 its freedom from alloys or impurities, its specific gravity, its freedom 



