46 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



force (which has manifested itself in the decomposition of the water and 

 the oxidation of the zinc) into electrical ; but the electrical current may, 

 in its turn, be made to produce chemical decomposition ; and the amount 

 of this kind of change which it will effect, bears a precise correspon- 

 dence (cceteris parilus) with the amount of zinc which has undergone 

 oxidation in the galvanic cell. In like manner, when water at 212 is 

 converted into steam, the heat which it receives is no longer manifested 

 as heat, but mechanical force is developed in its stead, and this in a 

 certain definite ratio, so that the ' mechanical equivalent' of heat is capa- 

 ble of being exactly determined : so soon, however, as the steam, losing 

 its elasticity by condensation, returns to the condition of water, the 

 original equivalent of heat is again developed, its mechanical force being 

 no longer manifested.* 



55. Now in every case in which one force is thus converted into 

 another, the change is effected through the medium of a certain form of 

 matter, or material substratum. This may be, in some cases, of almost 

 any description whatever ; as when Heat is produced by the friction or 

 retarded motion of solids, liquids, or even gases ; or when Motion (as 

 shown in expansion) is produced by the application of heat to any kind 

 of material substance. But in other cases, the change can only be 

 effected through some special form of matter ; or if several substances 

 may serve as its medium, there is some one which is greatly superior to 

 all the rest, in the readiness with which a certain force manifests itself 

 through it. Thus iron is the only substance through which Electricity 

 can be converted into Magnetism ; and the development of magnetic 

 force, therefore, can only take place through this medium. So, Heat 

 is more readily converted into Electricity through a combination of bis- 

 muth and antimony, than through any other metals ; and the affection 

 of Light by magnetic force (discovered a few years since by Prof. Fara- 

 day), though producible through any transparent substance, can be made 

 much more obvious when the magnetism is made to act upon a peculiar 

 glass composed of vitrified borate of lead, than through the medium of 

 any other substance yet known. This speciality in the action of diffe- 

 rent substances, when subjected to the same forces, is a fact of funda- 

 mental importance ; and it is on it, indeed, that our notion of their 

 several properties depends. 



56. Now as the properties of every kind of matter require certain 

 conditions for their manifestation, our acquaintance with them entirely 

 depends upon whether the conditions of their action have been afforded. 

 Thus, to go back to a former illustration, supposing a new chemical 



* The above statement is an expression of the simple facts of the case, which, when 

 thus understood, render the hypothesis of " latent heat" altogether unnecessary. This 

 hypothesis, however ingenious, will doubtless share the fate of many other such 

 attempts to substitute a form of words for realities. It supposed the 966 degrees of 

 heat expended in converting a certain amount of water at 212 into steam at 212, to 

 become altogether inactive or latent ; and gave no account whatever of the mechanical 

 force which is produced in that act of conversion. The idea of an inactive force, in fact, 

 is one that cannot be entertained ; for if a force ceases to be active, it is no longer 

 'force.' And it cannot be imagined that force, any more than matter, should cease to 

 exist ; it must manifest itself under some other aspect. For a complete exposition of 

 the mutual relations existing among the above-named agents, see Prof. Grove's treatise 

 " On the Correlation of the Physical Forces." 



