552 Sir Oliver Lodge n the 



o 



submerge the velocity of light by calling it 1, — really 1 centim. per unit. 

 (For of course it retains its dimensions and is not merely a number — 

 neither 1 nor any other number — though it runs the risk of being so 

 treated, or even forgotten altogether, in spite of its importance ; which 

 is so great that it is one of the few absolute things that relativists have 

 allowed themselves to retain.) The abolition of gravity makes the con- 

 vention easy that G shall equal 1 too, and then mass becomes a length, 

 in so far as length survive.?, and is also identified with energy. In fact 

 the units to which we are accustomed get purposely muddled, without 

 really proving any kind of identity such as is evidently hoped for by 

 some writers. 



If light has weight in any real sense it is tempting to 

 treat it as a gas of low molecular weight and great velocity, 

 for since its pressure is equal to the energy per c.c, p — ipc 2 , 

 it obeys Boyle's law like a gas of uniform very high tempe- 

 rature, the ^c 2 taking the plnce of ^ir appropriate to the 

 mean square speed of molecular motions ; for tlie 3 in gas- 

 theory has reference to the three dimensions of space, which 

 are not applicable to the one- or two-dimensional motion of 

 light. 



The density of solar radiation at the earth's distance can 

 be estimated from the measured pressure as of the order 

 10~ 25 gramme per c.c. 



At any other place, if the gravitational potential is V and 

 the gas constant p/p = k, the density should be obtainable 

 from v 



p = Ae~ k 

 if it is like one of the atmospheric gases. 



Now none of what we have been reckoning feels as if it 

 were at all likely to happen. It is not likely that the speed 

 •of light escaping from say a spiral nebula and managing to 

 reach the earth has been so much retarded en route that it 

 arrives with a speed below the normal. It is unlikely that 

 a ray travels quicker as it approaches a gravitative mass. 

 What has been established by observation is that passing- 

 rays are deflected towards such a muss, the effect of 

 gravitation being therefore to reduce the speed below the 

 normal, never to increase it. 



For the deflexion is as if space, or rather ether, had 

 the refractive index l + 2V/c 2 in regions of gravitational 

 potential V ; and, if that is a right way of expressing the 

 fact, light must travel slower as it approaches a mass, not 

 at all as if it had weight but as if it were entering a denser 

 medium. The retarding influence of matter is apparently 

 felt before the substance is actually entered, as if matter 



