and therefore, of course, there will be an outcry against any one who undertakes the necessary task of pointing out his real demerits. However, there is no such thing in scientific history as the argumentum ad misericordiam. The blame, if any there be in such a matter, is due to those who preposterously gave him credit for what he did not do. The real merits of Mayer, however, which are extremely great, but which are in danger of being forgotten or ignored in consequence of the unwarrantable claims made for him, depend upon his having, after getting a true theory by false reasoning from inadequate and sometimes inadmissible premises, reasoned rightly upon it, and developed it widely in its applications. Language has lost all meaning, however, if this can be called a claim to establishment of the theory itself. The fact is that in 1839 Faraday, and in 1841 Liebig, and about the same time others of the great philosophers who have lately died, made close approaches to the true theory by methods far more sound than those of either Mayer or Séguin; and yet, curiously enough, they have scarcely at any hand got the slightest recognition.1

The true modern originators and experimental demonstrators of the conservation of energy in its generality were undoubtedly Colding of Copenhagen and Joule of Manchester. It is interesting to see in what light these men regard Mayer and some others of those who preceded them. I shall presently give you a quotation or two bearing on that point.

In the meantime I may say, with regard to Colding,2 that he began by being metaphysical, but saw at once,

1 See Phil. Mag. 1864, II. p. 474; 1865, I. p. 217; and 1876, II. p. 110. 3 See his very interesting letter, Phil. Mag. Jan. 1864.

or very soon, that metaphysics was not the proper basis on which to found a search for physical facts. His metaphysics led him to form certain opinions, but before publishing one of them he set to work and laboriously brought it to the test of fact. Joule, on the other hand, seems to have begun by experimenting with the view of determining certain physical constants. He does not tell us whether he had any metaphysical opinion about their relations or not. He set to work experimenting, and it was only after a great and varied series of his experiments had been fully carried out, and valuable results obtained, that he began to make certain applications of metaphysical reasoning to the connections which he had discovered. He did not apply metaphysics to discover anything, but to try and co-ordinate with other things the discoveries he had already made. Colding's work is by no means so extensive as Joule's. It is very nearly simultaneous with it, but it is neither so exact nor so extensive. Still, although Colding is hardly to be compared with Joule, he stands enormously high in comparison with any of the others who had experimented up to that time upon the conservation of energy. I will read you one or two extracts from Colding, and you will see from them how properly he went to work. He says:

'It was in accordance with this idea that I twenty years ago presented to the Royal Society of Science here in Copenhagen, a treatise in which I explained my idea that force is imperishable and immortal; and, therefore, when and wherever force seems to vanish in performing certain mechanical, chemical, or other work, the force then merely undergoes a transformation and reappears in a new form, but of the original amount as an active force.

'In the year 1843 this idea, which completely constitutes the new principle of the perpetuity of energy, was distinctly given by me,

the idea itself having been clear to my own mind nearly four years before, when it arose at once in my mind by studying D'Alembert's celebrated and successful enunciation of the principle of active and lost forces; but of course the new principle was not as clear to me from the beginning as it was when I wrote my treatise in 1843.'

I may here parenthetically observe that Colding speaks of D'Alembert's celebrated and successful enuntiation of a certain principle. This is nothing more or less than a particular case of that principle of Newton, which I gave you in a former lecture; so that you see Colding really got his idea suggested to him by Newton's work:

'According to the view which led me to this principle, its future importance, in case it were really true, was perfectly clear to me from the first instant. But this made me very anxious not to publish it as a new law of nature until I should be able to give experimental proof of its truth; and scientific men to whom I explained my idea, and especially our celebrated professor, H. C. Ersted, agreed with me and advised me to be safe in this respect before I wrote; and it was for this reason that I departed from my original intention of explaining it to a meeting of Natural Philosophers held in Copenhagen in 1840.

'In my first treatise, of 1843, the title of which is "Theses concerning Force" (Nogle Sætninger om Kræfterne), I therefore not only presented my idea to the Royal Society (of Copenhagen) as a thing that most likely would hereafter be found to be a general law of nature, but, after stating that the only trustworthy decision of the question was to be got from the experimental investigation of nature itself, I went on to call attention to several old experiments made previously to my time, the first of which was Dulong's celebrated discovery respecting the heat disengaged or absorbed during the compression or expansion of a great number of different airs and gases, and I then showed how perfectly these experiments proved the truth of the said principle for bodies of that kind.'

1 Ante, p. 33. See Thomson and Tait's Natural Philosophy, § 264.

Then he goes on to say that having established the proposition for elastic fluids, he proceeded to try experiments in conjunction with Ersted upon the compression of water; and that next he advanced, just as Joule did about the same time, to experiments upon the compression of solids. He also says:

I closed my discussion by showing that the discovery of a perpetuum mobile would be possible if my principle was wrong.'

This shows that, to a certain extent at least, he had anticipated Helmholtz, of whose great services to this branch of science I shall presently speak.

The remarks he makes about Mayer deserve to be quoted. He desires the republication, in an English journal, of his first paper, in order that it might be compared, as he says, with the paper of Mayer, which was most loudly vaunted in England at the time when his letter was written :

'I need scarcely say that such a comparison would be of great interest to me, as I believe it would convince your readers of the fact that M. Mayer wrote his remarks in 1842, before he was able to support them by a single experiment or by anything like a proof of their exactness, whilst I thought it to be my duty, before I wrote, to prove that my suppositions concerning the forces were confirmed by nature itself, as a law of nature.’

He also says of his own experimental approximation to the dynamical equivalent of heat, that it is

'very near the proportion that M. Mayer in 1842 supposed, but did not prove, to be right.'

Joule's remarks1 upon the subject of Séguin and Mayer are also deserving of quotation :—

'Séguin gives data from which the mechanical equivalent of heat may be readily deduced on his hypothesis, the result being too 1 Phil. Mag. 1864, II. p. 151; see also 1862, II. p. 121.

great in consequence of the thermal effect of the compression of vapour being understated. Neither in Séguin's writings of 1839, nor in Mayer's paper of 1842, were there such proofs of the hypothesis advanced as were sufficient to cause it to be admitted into science without further inquiry. I believe that the experiment attributed to Gay-Lussac was not referred to by Mayer previously to the year 1845. Mayer appears to have hastened to publish his views for the express purpose of securing priority. He did not wait until he had the opportunity of supporting them by facts. My course, on the contrary, was to publish only such theories as I had established by experiments calculated to commend them to the scientific public, being well convinced of the truth of Sir J. Herschel's remark, that “hasty generalisation is the bane of science.""

To these it would be easy to add several even more telling passages to the same effect.

[In 1876 my attention was called to a paper by Mohr (Journal für Pharmacie), of which I published a translation in the Phil. Mag. for August of that year. The date of the paper is 1837, or five years before Mayer, and it contains, in a considerably superior form, almost all that is correct in Mayer's paper. Though it contains many mistakes, it avoids some of the worst errors of Mayer, especially his false analogy and his à priori reasoning. The very process (for determining the mechanical equivalent of heat from the two specific heats of air) for which Mayer has been so extravagantly lauded:—although it is in principle, albeit not in practice, utterly erroneous :- -is here stated much more clearly than it was stated five years later by Mayer.

In December 1877, I received by post a copy of a work Allgemeine Theorie der Bewegung und Kraft, etc. (Braunschweig 1869), with the inscription, in a bold hand, 'dedicated by the author, Dr. Mohr.' This work is conclusive against Mayer's first paper. It leaves absolutely