324 THE PKINCIPLES OF SCIENCE. [CHAP. 



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

 density. To effect this reduction, however, in any particu- 

 lar 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 increases, we can easily determine 

 their comparative brilliance. Hence we can express the 

 intensity of light falling upon any surface, if we have a 

 unit in which to make the expression. Light is un- 

 doubtedly one form of energy, and the unit ought therefore 

 to be the unit of energy. But at present it is quite im- 

 possible 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 assign its 

 relation to other forms of energy ? If we answer Yes, it is 

 equivalent to saying that the science of light must stand 

 still perhaps for a generation ; and not only this science 

 but many others. The true course evidently is to select, 

 as the provisional unit of light, some light of convenient 

 intensity, which can be reproduced from time to time in 

 the same intensity, and which is defined by physical cir- 

 cumstances. All the phenomena of light may be experi- 

 mentally investigated relatively to this unit, for instance 

 that obtained after much labour by Bunsen and Roscoe. 1 

 In after years it will become a matter of inquiry what is 

 the energy exerted in such unit of light ; but it may be 

 long before the relation is exactly determined. 



A provisional unit, then, means one which is assumed 

 and physically defined in a safe and reproducible manner, 

 in order that particular quantities may be compared inter 

 se more accurately than* they can yet be referred to the 

 primary units. In reality the great majority of our 

 measurements are expressed in terms of such provisionally 

 independent units, and even the unit of mass, as we have 

 seen, ought to be considered as provisional. 



The unit of heat ought to be simply the unit of energy, 

 already described. But a weight can be measured to the 



1 Philosophical Transactions (1859), vol. cxlix. p. 884, &c. 



