UNITS AND STANDARDS OF MEASUREMENT. 377 



quantities in terms of any unit which he likes to adopt. 

 He may use the yard for linear measurement and the 

 litre for cubic measurement, only there will then be a 

 complicated relation between his different results. The 

 system of derived units which we have been briefly con- 

 sidering, is that which gives the most simple and natural 

 relation between quantitative expressions of different 

 kinds, and therefore conduces to ease of comprehension 

 and saving of laborious calculation. 



Provisionally Independent Units. 



Ultimately, as we can hardly doubt, all phenomena 

 will be recognised as so many manifestations of energy ; 

 and, being expressed in terms of the unit of energy, will 

 be referable to the primary units of space, time, and 

 mass. To effect this reduction, however, in any particular 

 case, we must not only be able to compare different 

 quantities of the phenomenon, but to trace the whole 

 series of steps by which it is connected with the primary 

 notions. We can readily observe that the intensity of 

 one source of light is greater than that of another ; and, 

 knowing that the intensity of light decreases as the 

 square of the distance, we can easily determine their 

 comparative brilliance. Hence we can express the inten- 

 sity of light falling upon any surface, if we have a unit 

 in which to make the expression. Light is undoubtedly 

 one form of energy, and the unit ought therefore to be 

 the unit of energy. But at present it is quite impossible 

 to say how much energy there is in any particular 

 amount of light. The question then arises, Are we to 

 defer the measurement of light until we can fully and 

 accurately assign its relation to other forms of energy I 

 If we answer Yes, it is equivalent to saying that the 

 science of light must stand still perhaps for a generation ; 



