OF VITAL FORCE IN GENERAL. 45 



and the laws of Optics are expressions of the conditions under which 

 that force is propagated, and of its action on material substances. In 

 Thermotics, again, we have to do with the force of Heat ; and its laws 

 are expressions of the circumstances under which heat is propagated, 

 and of the changes which it occasions in the substances it affects. So in 

 the sciences of Electricity and Magnetism, we have to do with the forces 

 known under those names ; and with the laws expressive of their action" 

 upon matter. And the scientific Chemist refers all the phenomena with 

 which he is concerned to the operations of Chemical Affinity, and endea- 

 vours to deduce from observation of the phenomena the laws of the 

 operation of this force. So the Physiologist will be justified in assuming 

 a Vital Force (or Forces) as the power which operates in producing 

 Vital phenomena ; and will most legitimately pursue his science, in 

 inquiring into the conditions under which that force operates. 



53. The analogy of the Physical Sciences may be advantageously 

 pursued further. Although we are accustomed to speak of the power 

 that produces Mechanical Motion, of Light, of Heat, of Electricity, of 

 Magnetism, and of Chemical Affinity, as distinct forces, yet it has gra- 

 dually become apparent that very intimate relations subsist between 

 them, and that they are, in fact, mutually convertible ; so that one force 

 (A) operating upon a certain form of matter, ceases to manifest itself, 

 but developes another force (B), in its stead ; whilst, in its turn, the 

 second force (B) may be reconverted into the first (A), or into some other 

 (c), which, again, may reproduce either the first (A), or second (B), or 

 some other (D or B). It was in the case of Electricity and Magnetism, 

 that this reciprocal relation, which is designated as ' correlation,' was 

 first clearly apprehended. If an electric current be passed round a piece 

 of soft iron, that iron becomes magnetic, and remains so as long as the 

 current is circulating : on the other .hand, from a magnet put in motion, 

 an electric current may be obtained. Hence we are accustomed to con- 

 nect these two forces under the term Electro-Magnetism ; but they can 

 be easily shown to be quite distinct in their modes of operation on matter ; 

 and their relation is not really more intimate than that of other forces. 

 For Heat may be developed by Electricity ; as when a galvanic current, 

 sent through a thin platinum wire, heats it to ignition, or even fuses it. 

 Conversely, Electricity may be developed by Heat ; as when heat is 

 applied to bars composed of dissimilar metals in contact with each other. 

 Again, if Mechanical Motion be retarded, as in friction, we immediately 

 have a development either of Heat or of Electricity ; heat alone being 

 developed, when the two rubbing surfaces are composed of precisely the 

 same substance ; and electricity being produced, when these substances 

 are different. And it is for the most part through the medium of Heat 

 or Electricity, that the force of Mechanical Motion is < correlated' to 

 Light, Magnetism, and Chemical Affinity. 



54. The idea of correlation also involves that of a certain definite ratio, 

 or relation of equivalence, between the two forces thus mutually inter- 

 changeable ; so that the measure of force B, which is excited by a certain 

 exertion of force A, shall, in its turn, give rise to the same measure of 

 force A, as that originally in operation. Thus, when an electric current 

 is set in motion by galvanic action, we have a conversion of chemical 



