328 PROCEEDINGS OF THE AMERICAN ACADEMY. 



1 joule (1 watt-second, or 10'^ ergs). Here the unit of capacity will 

 vary with each new increase in accuracy in the determination of the 

 mechanical equivalent ; but capacity is a less tangible dimension than 

 temperature, and its variation would cause less instrumental confusion. 



This convenient unit of capacity would be nearly represented by the 

 heat capacity of a gram of magnesium at low temperatures (—50°), or 

 that of a gram of aluminum at high temperatures (about 290° C). At 

 ordinary temperatures an alloy of zinc and magnesium containing about 

 5.5 per cent of zinc would probably have the desired capacity. 



Specific heats are frequently spoken of in terms of calories, thus con- 

 founding heat capacity with energy. As a matter of fact, the idea of 

 specific heat is mathematically nothing but a simple ratio like specific 

 gravity, — a pure number without physical dimensions. The unit sug- 

 gested here is rather to be compared with density ; it has the definite 

 dimension of energy divided by temperature. 



It seems to me that a name for this unit would greatly assist the 

 beginner to discriminate between energy and capacity. Would not the 

 name " mayer," in honor of the unfortunate Julius Robert Mayer, one 

 of the discoverers of the first law of energy, be a convenient and fitting 

 term for the centimeter-gram-second -r- centigrade unit of heat capacity ? 



On this basis the heat capacity of a gram of water at twenty degrees 

 centigrade is about 4.181 mayers, and that of a gram of liquid mercury 

 is .0333 X 4.18 = 0.139 mayers. The gas constant becomes 8.32 

 mayers, if the atomic weight of oxygen is taken as ] 6 ; and the Dulong 

 and Petit constant or gram-atomic heat capacity becomes about 26.5 

 mayers on the same basis. These numbers are all of convenient magni- 

 tude. For larger values, such as the heat capacities of solutions used in 

 thermochemistry, the kilomayer is a convenient unit. F'or instance, the 

 capacity of HCl + 100 HoO is 7.41 kilomayers, while that of a similarly 

 dilute solution of 40 grams (a mol) of sodic hydroxide is 7.42 kilomayers. 

 The solution produced by mixing these two has a capacity of 15.02 kilo- 

 mayers. In order to show how convenient these figures are as a basis 

 of calculation, it is only necessary to point out that this difference of 0.19 

 kilomayer between the capacities of factors and product indicates that the 

 heat of neutralization will vary 0.19 kilojoule * for each degree of tem- 



* Ostwald lias pointed out the convenience of the kilojoule as a unit in thermo- 

 chemistry, in the latest edition of the "Grundriss der allgemeinen Cliemie" It 

 seems to me that it would be well to represent this useful unit by kj., in analogy 

 to km. and kg., ratlier than by J., which might be mistaken as an abbreviation 

 for joule. Kilomayer maybe abbreviated to kmy. 



