Dec. 14, 1871] 



NATURE 



119 



Thus, in baldest outline, I have sought to give some 

 notion of the first half of this marvellous essay. The 

 second half is so exclusively physiological that I do not 

 wish to meddle with it. I will only add the illustration 

 employed by Mayer to explain the action of the nerves 

 upon the muscl:s. As an engineer, by the motion of his 

 finger in opening a valve or loosing a detent, can liberate 

 an amount of mechanical motion almost infinite compared 

 with its exciting cause, so the nerves, acting upon the 

 muscles, can unlock an amount of activity wholly out of 

 proportion to the work done by the nerves themselves. 



As regards these questioas of weightiest import to the 

 science of physiology. Dr. Mayer in 1845 was assuredly 

 far in advance of all living men. 



Mayer gr.isped the mechanical theory of heat with 

 commanding power, illustrating it and applying it in the 

 most diverse domains. He began, as we have seen, v/ith 

 physical principles ; he determined the numerical relation 

 between heat and work ; he revealed the source of the 

 energies of the vegetable world, and showed the relation- 

 ship of the heat of our fires to solar heat. He followed 

 the energies which were potential in the vegetable up to 

 their local exhaustion in the animal. But in 1845 

 a new thought was forced upon him by his calculations. 

 He then for the first time drew attention to the astound- 

 ing amount of heat generated by gravity where the force 

 has sufficient distance to act through. He proved, as I 

 have before stated, the heat of collision of a body failing 

 from an infinite distance to the earth, to be sufficient to 

 raise the temperature of a quantity of water equal to the 

 falling body in weight I7,356°C. He also found in 1845 

 that the gravitating force between the earth and sun 

 was competent to generate an amount of heat equal 

 to that obtainable from the combustion of 6,000 times the 

 weight of the earth of solid coal. With the quickness ot 

 genius he saw that we had here a power sufficient to pro- 

 duce the enormous temperature of the sun, and also to 

 account for the primal molten condition of our own planet. 

 Mayer shows the utter inadequacy of chemical forces, as 

 we know them, to produce or maintain the solar tempera- 

 ture. He shows that were the sun a lump of coal, it would 

 be utterly consumed in 5,000 years. He sho'.vs the difli- 

 culties attending the assumption that the sua is a cooling 

 body ; for supposing it to possess the high specific heat 

 of water, its tem.perature would fall 15,000° in 5,000 

 years. He finally concludes that the light and heat 

 of the sun are maintained by the constant impact of 

 meteoric matter. I never ventured an opinion as to the 

 accuracy of this theory ; that is a question which may still 

 have to be fought out. But I refer to it as an illustration 

 of the force of genius with wh'ch Mayer followed the 

 mechanical theory of heat through all its applications. 

 Whether the meteoric theory be a matter of fact or not, 

 with him abides the honour of proving to demonstra- 

 tion that the light and heat of suns and stars may be 

 originated and maintained by the collisions of cold 

 planetary matter. 



It is the man who from the scantiest data could accom- 

 plish all this in six short years, and in the hours snatched 

 from the duties of an arduous profession, that the Royal 

 Society has this year crowned with its highest honour. 

 Dr. Mayer had never previously received any mark of 

 recognition from the society. 



It was not in my power to be present at our late presi- 

 dent's last address ; but Sir Edward Sabine has done me 

 the honour of sending me a printed copy of it. It con- 

 tains the reasons assigned by him for the award of the 

 Copley medal. Briefly, but appreciatingly, he expresses 

 his opinion of the merits of Dr. Mayer, committing to Prof. 

 Stokes the task of drawing up a fuller statement of the case. 

 This statement is marked by an evident desire to act fairly 

 towards Mayer, and at the same time to qualify the award 

 so that no erroneous inferences may be drawn from it. 

 It will be observed that Prof. Stokes confines himself to 

 Mayer's first paper, the real value of which, however, is 

 best appreciated in connection with Mayer's subsequent 

 work, as the soundness of the root is best demonstrated 

 by the vigour of the tree. Prof. Stokes writes thus : — 



" In a paper published in 1842, Mayer showeJ that he 

 clearly conceived the convertibility of falling force, or of 

 the vis viva, which is its equivalent or representative in 

 visible motion, into heat, which again can disappear as 

 heat by reconversion into work or vis viva, as the case 

 may be. He pointed out the mechanical equivalent of 

 heat as a fundamental datum, like the space through 

 which a body falls in one second, to be obtained from 

 experiment. He went further. When air is condensed 

 by the application of pressure, heat, as is well known, is 

 produced. Taking the heat so produced as the equivalent 

 of the work done in compressing the air, Mayer obtained 

 a numerical value of th=^ mechanical equivalent of heat, 

 which, when corrected by employing a more precise value 

 of the specific heat of air than that accessible to Mayer, 

 does not much differ from Joule's result. This was un- 

 doubtedly a bold idea, and the numerical value obtained 

 by Mayer's method is, as we now know, very nearly 

 correct." Prof. Stokes then qualifies the award in these 

 words : — " Nevertheless it must be observed that an 

 essential condition in a trustworthy determination is 

 wanting in Mayer's method ; the portion of matter 

 operated on does not go through a cycle of changes. 

 Mayer reasons as if the production of heat were the sole 

 effect of the work done in compressing air. But the 

 volume of the air is changed at the same time, and it is 

 quite impossible to say a priori whether this change may 

 not involve what is analogous to the statical compression 

 of a spring, in which a portion or even a large portion of 

 the work done in compression may have been expended. 

 In that case the numerical result given by Mayer's 

 method would have been erroneous, and anight have been 

 even widely erroneous. Hence the practical correctness 

 of the equivalent obtained by Mayer's method must not 

 lead us to shut our eyes to the merit of our own country- 

 man Joule, in being the first to determine the mechanical 

 equivalent of heat by methods which are unexceptionable, 

 as fulfilling the essential condition that no ultimate change 

 of state is produced in the matter operated upon." 



The Judgment of Prof. Stokes, regarding the possible 

 error of Mayer's determination of the mechanical equi- 

 valent of heat, gives me occasion to cite another proof 

 of the insight of this extraordinary man. His paper of 

 1S45 contains the details of his calculation, which were 

 omitted from his first brief paper. Mayer prefaces the 

 calculation with these memorable words :— 



"To prove this important proposition, we must fix 

 our attention on the deportment of elastic fluids towards 

 heat and mechanical eftect. 



