122 ATOMS AND SUNBEAMS. 



account for tlie extremely remarkable circumstance that our great 

 luminary has radiated forth already a thousand times as nuich heat as 

 could be generated by the combustion of a sphere of coal as big as the 

 sun is at present, and yet, notwithstanding this expenditure in the 

 past, physics declares that for millions of years to come the sun nmy 

 continue to dispense light and heat to its attendant worlds with the 

 same abundant prodigality. To have shown how the apparent para- 

 dox could be removed is one of the most notable achievements of the 

 great German philosopher. 



What Helmoltz did was to refer to the obvious fact that the expendi- 

 ture of heat by radiation must necessarily lead to shrinkage of the 

 solar volume. This shrinkage has the effect of abstracting from a store 

 of potential energy m the sun and transforming what it takes into the 

 active form of heat. The transformation advances pari passu with the 

 radiation, so that the loss of heat arising from the radiation is restored 

 by the newly produced heat derived from the lateut reservoir. Such is 

 an outline of the now famous doctrine universally accepted among 

 physicists. It fulfils the conditions of the problem, and when tested 

 by arithmetical calculation it is not found wanting. 



But the genuine student of nature loves to get to the heart of a great 

 problem like this. He loves to be able to follow it, not through mere 

 formulic or abstract j)rinciples, but so as to be able to visualise its truth 

 and feel its certainty. He will tlierefore often desire something in 

 addition to the bare presentation of the theory as above stated. It 

 may be no doubt sufiicient for the mathematician to know that the total 

 potential energy in the sun, due to the dispersed nature of its materi- 

 als, is so vast that as contraction brings the materials, on the whole, 

 somewhat nearer together, the potential energy thus surrendered is 

 transformed into a supply of heat (piite adequate to compensiite for the 

 losses arising from the radiation by which the contraction was pro- 

 duced. The student who admits — and who is there that does not 

 admit? — the doctrine of the conservation of energy, knows that in this 

 argument he is on thoroughly reliable ground. At the same time the 

 argument does not actually offer any very clear conception, or indeed 

 any conception at all, of the precise modus operandi by which, as the 

 active i)oteiitial energy vanishes, its equivalent in available heat 

 appears. I have always felt that this was the unsatisfactory part of 

 an otherwise perfect theory. It was therefore with much interest that 

 I became acquainted a short time ago with a develo}»mentof the mole- 

 cular theory of gases, which afforded precisely what seemed wanted to 

 make every link in the chain of the great argument distinctly jiercepti- 

 ble. I make no doubt that the notions which have occurred to me on 

 this subject must have presented themselves to others also. I have 

 however not read in print or heard in conversation any use made of the 

 illustration that I am going to set forth. I feel therefore confident 

 that even if it be known at all, it is certainly not generally known 



