104 POPULAR SCIENCE MONTHLY 



of solar energy by radiation, the sun could acquire its present energy 

 of rotation in thirty-two thousand years. From an estimate of the 

 limited amount of meteoric matter available near the sun, he concludes 

 that " sunlight can not last as at present for three hundred thousand 

 years." This calculation of the year 1854 attracted no attention at 

 the time. Later, the theory was abandoned by its author, as at vari- 

 ance with known facts. 



Evidently the theory of solar heat was still in a very crude form. 

 But important new ideas were brought into view in the same year, 

 1854, by Helmholtz, in a popular lecture at Konigsberg, delivered on 

 the occasion of the Kant commemoration. 22 Unlike Mayer and Thom- 

 son, he starts out with the nebular hypothesis of Kant and Laplace, 

 and derives solar heat from nebular contraction. During the contrac- 

 tion of the nebula from which sun and planets were formed, and also 

 during the contraction of the sun, now assumed to be in progress, the 

 kinetic energy obtained thereby is converted into heat and compensates 

 for the loss of solar heat by radiation. He concludes that if the sun 

 contracts the ten-thousandth part of its radius enough heat is gen- 

 erated to supply radiation for 2,100 years. 23 His figures yield twenty- 

 two million years as the probable age of the sun, on the assumption of 

 uniform radiation and homogeneous density. Experimental data on 

 the intensity of solar radiation, found later by Langley, reduced this 

 age to eighteen million or less. 



Helmholtz's theory was a tremendous advance on that of falling 

 meteors, assumed by Mayer and Thomson. No doubt meteors fall into 

 the sun, as assumed by Mayer and Thomson, but the Mayer-Thomson 

 theory made demands upon these meteors that bordered on extrava- 

 gance. We are certain that a part of the solar heat is due to falling 

 meteors, but its amount is as nothing, compared with the heat result- 

 ing from the gravitational energy of shrinkage. Until recently these 

 were the only important sources considered. 



In the sixties fresh attacks were made on the problem of the age of 

 the sun by William Thomson. In 1862 appeared in the Macmillan's 

 Magazine an article, " On the Age of the Sun's Heat," 24 in which he 

 favors a meteoric theory like that of Helmholtz, by which there is no 

 difficulty in accounting for 20,000,000 years' heat radiated by the sun. 

 He concludes that we may accept " as a lowest estimate for the sun's 

 initial heat, 10,000,000 times a year's supply at present rate, but 50,- 

 000,000 or 100,000,000 as possible, in consequence of the sun's greater 



22 H. Helmholtz, "Popular Lectures" (transl. by E. Atkinson), New York, 

 1897, pp. 153-193, " On the Interaction of Natural Forces." 



23 Op. cit., p. 190. 



21 Sir William Thomson's " Popular Lectures and Addresses," Vol. I., 1891, 

 pp. 356-375. 



