360 3Ir. Faraday [Feb. 27, 



greatest simplicity ; and precisely for this reason, that gravitation 

 has not yet been connected by any degree of convertibility with the 

 other forms of force. If I refer for a few minutes to these other 

 forms, it is only to point in their variations, to the proofs of the 

 value of the principle laid down, the consistency of the known 

 phenomena with it, and the suggestions of research and discovery 

 which arise from it.* Heat, for instance, is a mighty form of 

 power, and its effects have been greatly developed ; therefore, 

 assumptions regarding its nature become useful and necessary, and 

 philosophers try to define it. The most probable assumption is, 

 that it is a motion of the particles of matter ; but a view, at one 

 time very popular, is, that it consists of a particular fluid of heat. 

 Whether it be viewed in one way or the other, the principle of 

 conservation is admitted, I believe, with all its force. When trans- 

 ferred from one portion to another portion of like matter the full 

 amount of heat appears. When transferred to matter of another 

 kind an apparent excess or deficiency often results; the word 

 " capacity " is then introduced, which, whilst it acknowledges the 

 principle of conservation, leaves space for research. When em- 

 ployed in changing the state of bodies, the appearance and disap- 

 pearance of the heat is provided for consistently by the assumption 

 of enlarged or diminished motion, or else space is left by the term 

 " capacity '* for the partial views ; which remains to be developed. 

 When converted into mechanical force, in the steam or air-engine, 

 and so brought into direct contact with gravity, being then easily 

 placed in relation to it, still the conservation of force is fully re- 

 spected and wonderfully sustained. The constant amount of heat 

 developed in the whole of a voltaic current described by M. P. A. 

 Favre,t and the present state of the knowledge of thermo-electricity, 

 are again fine partial or subordinate illustrations of the principle of 

 conservation. Even when rendered radiant, and for the time giving 

 no trace or signs of ordinary heat action, the assumptions regarding 

 its nature have provided for the belief in the conservation of force, 

 by admitting, either that it throws the ether into an equivalent state, 

 in sustaining which for the time the power is engaged ; or else, that 

 the motion of the particles of heat is employed altogether in their 

 own transit from place to place. 



It is true that heat often becomes evident or insensible in a 

 manner unknown to us ; and we have a right to ask what is hap- 

 pening when the heat disappears in one part, as of the thermo- 

 voltaic current, and appears in another ; or when it enlarges or 

 changes the state of bodies ; or what would happen, if the heat, 

 being presented, such changes were purposely opposed. We have 

 a right to ask these questions, but not to ignore or deny the con- 



* HelmhoUz, On the Conservation of Force. Taylor's Scientific Memoirs, 

 2nd Series, 1853, p. 114. 



t Comptes Rendus, 1854, Vol. xxxix., p. 1212. 



