UNITS AND STANDARDS OF MEASUREMENT. 383 



to reward the enormous labour which must be undertaken 

 in these calculations. Such labours once successfully com- 

 pleted must benefit the whole human race as long as it 

 shall exist. A valuable account of all the chief mathe- 

 matical tables yet published will be found in De Morgan's 

 article on Tables, in the 'English Cyclopaedia/ Division 

 of Arts and Sciences, vol. vii. p. 976. 



Physical Constants. 



The second class of constants contains those which refer 

 to the actual constitution of matter. For the most part 

 they depend upon the peculiarities of the chemical sub- 

 stance in question, but we may begin with those which 

 are of the most general character. In a first sub-class 

 we may place the velocity of light or heat undulations, 

 the numbers expressing the relation between the lengths 

 of the undulations, and. the rapidity of the undulations, 

 these numbers depending only on the properties of the 

 ethereal medium, and being probably the same in all parts 

 of the universe. The theory of heat gives rise to several 

 numbers of the highest importance, especially Joule's 

 mechanical equivalent of heat, the absolute zero of tempe- 

 rature, the mean temperature of empty space, &c. 



Taking into account the diverse properties of the 

 elements we must have tables of the atomic weights, 

 the specific heats, the specific gravities, the refractive 

 powers, not only of the elements, but their almost in- 

 finitely numerous compounds. The properties of hardness, 

 elasticity, viscosity, expansion by heat, conducting powers 

 for heat and electricity, must also be determined in 

 immense detail. There are, however, certain of these 

 numbers which stand out prominently because they serve 

 as intermediate units or terms of comparison. Such are, 

 for instance, the absolute coefficients of expansion of air, 



