MEMOIR OF KIRCHHOFP. 587 



cause cau be produced ; the observed fact may be exi»laii»ed if it be ad 

 initted that the rays of liglit that make the sohir spectrum have trav- 

 ersed vapors of irou and suffered au absori)tion such as vapors of iron 

 generally produce. It is at the same time the only causi^ that can be 

 adduced; its adoption seems accordingly necessary." 



We may insert here a story that Kirchhoff liked to relate liimself. 

 The question whether Fraunhofer's lines reveal tlie presence of gold in 

 the sun was being investigated. Kirchhott's banker remarked on this 

 occasion : "What do I care for gold in the sun if I cau not fetch it down 

 liere?" Shortly afterwards Kirchhoff re(;eived from England a medal 

 for his discovery, and its value in gold. While handing it over to his 

 banker, he observed: "Look here, I have succeeded at last in fetching 

 some gold from the sun." As to Kirchhoff's own opinion of the impor- 

 tance of this law, it was quite indifit'ereut to him, as stated above, whether 

 the law admitted of any application to the investigation of the nature of 

 the sun and fixed stars, or had only a theoretical interest. As a char- 

 acteristic trait of him may be mentioned that in his theoretical lectures 

 he never says a single word about the region to which access was gained 

 through his discovery, and in his collected papers he grants it a place 

 only near the end. 



The other papers of Kirchhoff treat various subjects of mathematical 

 physics. Those concerned with electricity are the most numerous. A 

 whole series of them is devoted to the calculation of the paths the elec- 

 trical current takes in bodies of different shape or in a net- work of con- 

 duction. There is a Kirchhoflf's law about it too, which is of funda- 

 mental importance for the investigation of the distribution of the flow 

 of electricity in complicated conditions of conduction. Another series 

 of papers treats of the distribution of static electricity and magnetism. 

 These were in part celebrated problems on which the greatest of his 

 predecessors (like Poisson), had already tried their forces and had not 

 succeeded in mastering them so completely as Kirchhoff. He was the 

 first to apply the so-called mechanical theory of heat to chemical pro- 

 cesses, and by this application he bridged the way to the connection of 

 different branches of natural science by means of mechanical principles. 

 The basis of the mechanical theory of heat, the law of the permanency 

 of work, as Kirchhoff styled it, is according to him undoubtedly the 

 most important accession of knowledge gained in our century in the 

 region of natural science.* 



The brilliant, various, and apparently complicated phenomena of 

 light, Kirchhoff deduced in his lectures on o[)lics from the purely me- 

 chanical theory of an elastic body. That tether is such a l»ody is a hy[)oth- 

 esis which, though enunciated by Kirchhofit's predecessors, was worked 

 out by him in a particularly vigorous way. Nevertheless, all ])henomena 

 can not be explained by such a supposition. That Kirchhoff developed 

 this hypothesis and this only, and contented himself with mentioning at 



* His discourse as rector of H«M(lel1»erg University 1865. 



