276 PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



By another metliocl, Hopkins obtained the following results : 



Melting temperature. 



X. SPECIFIC HEAT IX THE ACCEPTATION OP THE MECHANICAL THEORY 



OF HEAT. 



By the ppecific heat of a substance is understood, we know, the number which 

 specifies how many units of heat (calories) must be added to the unit of Aveight 

 of tliat substance in order to produce an elevation of temperature from 0° to 1° C. 



According to a law propounded by Dalong and Petit, the product obtained 

 when the specific heat of a solid element is multiplied by its atomic weight should 

 be a constant number ; which is, indeed, nearly the case, as the following brief 

 table will show : 



Silver... 

 Alumina 

 Copper. - 



Iron 



Lithium. 

 Natrium. 



Lead 



Sulphur. 

 Ziuc. 



Atomic weight. 



108.0 

 27.4 

 G3. 4 

 56.0 

 7.0 

 23.0 



2U7. 

 32.0 

 Go. 2 



Specific heat. 



0. 0570 

 0.2143 

 0. 0949 

 0.1138 

 0. 9408 

 0. 2934 

 0.0314 

 0. 1776 

 0. 0956 



6.16 

 5.87 

 6.02 

 6.37 

 6.59 

 6.75 

 6.50 

 5.68 

 6.23 



The Dulong-Petit law admits of being expressed in this wise : ClicmicaTly 

 eqnivaleMt quantities of solid elements require for like elevation of temperature 

 quantities of heat of like amount. Still another expression of the same law is the 

 following : The atoms of all simple substances have a like capacity for heat; or, in 

 fine, the atomic heat of all simple suhstanc'es is equal, if we denote by atomic heat 

 the product of the atomic weight into the specific heat. 



In the mean, the atomic heat of solid elements has the product p • s=a, or 

 the value 6.4. From this value, however, the atomic heat of carbon deviates 

 considerably, since we have for — 



p • s. 



Charcoal 

 Graphite. 

 DiamonJ 



2.89 

 2. 09 

 1.76 



The atomic heat of the difibrent forms of carbon is somewhat more approxi- 

 mate to the mean value of the atomic heat of the rest of the elements, if, accord- 

 ing to lleguault's proposition, we take the atomic Aveight of carbon not as equal 

 to 12, but to 24. 



