162 Lord Kelvin on Ether and 



own atmosphere, its existence is a fact that cannot be ques- 

 tioned when the overwhelming evidence in favour of the 

 undulatory theory is considered ; and the investigation of 

 its properties in every possible way becomes an object of the 

 greatest interest. A first question would naturally occur, 

 What is the absolute density of the luminiferous ether in any 

 part of space ? I am not aware of any attempt having 

 hitherto been made to answer this question, and the present 

 state of science does not in fact afford sufficient data. It has, 

 however, occurred to me that we may assign an inferior limit 

 to the density of the luminiferous medium in interplanetary 

 space by considering the mechanical value of sunlight as 

 deduced in preceding communications to the Royal Society 

 [Trans. R. S. E. ; Mechanical Energies of the Solar System ; 

 republished as Art. LXVI. of Math, and Phys. Papers] from 

 Pouillet's data on solar radiation, and Joule's mechanical 

 equivalent of the thermal unit. Thus the value of solar 

 radiation per second per square centimetre at the earth's 

 distance from the sun, estimated at 1235 cm .-grams, is the 

 same as the mechanical value of sunlight in the luminiferous 

 medium through a space of as many cubic centimetres as the 

 number of linear centimetres of propagation of light per 

 second. Hence the mechanical value of the whole energy, 

 kinetic and potential, of the disturbance kept up in the space 

 of a cubic centimetre at the earth's distance from the sun*, is 



1235 412 „ 



3^l0ro' or ioro ofa ^-^ m - 



§ 2. The mechanical value of a cubic kilometre of sunlight 

 is consequently 412 metre-kilograms, equivalent to the work 

 of one horse-power for 5*4 seconds. This result may give 

 some idea of the actual amount of mechanical energy of the 

 luminiferous motions and forces within our own atmosphere. 

 Merely to commence the illumination of eleven cubic kilometres 

 requires an amount of work equal to that of a horse-power 

 for a minute ; the same amount of energy exists in that space 

 as long as light continues to traverse it ; and if the source 

 of light be suddenly stopped, must pass from it before the 

 illumination ceases f. The matter which possesses this energy 



* The mechanical value of sunlight in any space near the sun's surface 

 must be greater than in an equal space at the earth's distance, in the 

 ratio of the square of the earth's distance to the square of the sun's 

 radius, that is ; in the ratio of 46,000 to 1 nearly. The mechanical value 

 of a cubic centimetre of sunlight near the sun must, therefore, be 

 1236x40,000 or about . 0019 of a c am> 

 3xl0 10 ; 



t Similarly we find 4140 horse-power for a minute as the amount of 

 work required to generate the energy existing in a cubic kilometre of" 

 light near the sun. 



