Prof. Helmholtz on the Interaction of Natural Forces. 499 



His labours remained for a long time without notice, and it 

 was not till eighteen years afterwards, that is in 1842, that dif- 

 ferent investigators in different countries, and independent of 

 Caruot, laid hold of the same thought. The first who saw truly 

 the general law here referred to, and expressed it correctly, was 

 a German physician, J. R. Mayer of Heilbronn, in the year 

 1842. A httle later, in 1843, a Dane named Colding presented 

 a memoir to the Academy of Copenhagen, in which the same 

 law found utterance, and some experiments were described for 

 its further corroboration. In England, Joule began about the 

 same time to make experiments having reference to the same 

 subject. We often find, in the case of questions to the solution 

 of which the development of science points, that several heads, 

 quite independent of each other, generate exactly the same series 

 of reflections*. 



I myself, without being acquainted with either Mayer or 

 Colding, and having first made the acquaintance of Joule's ex- 

 periments at the end of my investigation, followed the same 

 path. I endeavoured to ascertain all the relations bettveen the 

 difi"erent natural processes, which followed from our regarding 

 them from the above point of view. My inquiry was made 

 public in 1847, in a small pamphlet bearing the title, " On the 

 Conservation of Force f." 



Since that time the interest of the scientific public for this 

 subject has gradually augmented, particularly in England, of 

 which I had an opportunity of convincing myself during a visit 

 last summer. A great number of the essential consequences of 

 the above manner of viewing the subject, the proof of which was 



* Tlie following extract is taken from a lecture b}' Mr. Grove, delivered 

 at the London Institution on the 19th of January, 1842: — 



" Light, heat, electricity, magnetism, motion, and chemical affinity, are 

 all convertible material affections ; assuming any one as the cause, one of 

 the others will be the effect. Thus heat ma)' be said to produce electricity, 

 electricity to produce heat; magnetism to produce electricity, electricity 

 magnetism ; and so of the rest. Cause and effect, therefore, in theii- rela- 

 tion to such forces, are words solely of convenience : we are totally unac- 

 quainted with the generating power of each and all of them, and probably 

 shall ever remain so : we can only ascertain the normal of their action : we 

 must humbly refer theu* causation to one omnipreseut influence, and con- 

 tent ourselves wit!, studying their effects, and developing by experiment 

 their nuitual relations." 



" I have long held an opinion," says Mr. Faraday in 1845, " almost 

 amounting to conviction, in common I believe with many other lovers of 

 natural knowledge, that the various forms under which the forces of matter 

 are made manifest have a common origin, or in other words, are so directly 

 related and mutually dependent, that they are convertible one into an- 

 other."— Tr. 



t A translation of this important essay appears in the Scientific Me- 

 moirs, New Series, p. 114. — J. T. 



