1 16 HEAT. 



It is usual to go somewhat further than the statement that the forms 

 of energy may replace each other this being all that experiment alone 

 would warrant us in saying and to regard these replacements as rather 

 transformations of one and the same thing energy ; which we suppose 

 to be identical with itself, though varying in its phenomenal appearance. 

 With this belief we may make the following statement : 



The Identity Of Energy. There is something which we term 

 " energy," and which may be recognised in various forms. When it dis- 

 appears in one form it appears in one or more other forms.* 



The Conservation or Constancy of Energy. So far we have 



considered energy only in its qualitative aspect, and have not considered 

 any mode of measuring it except in the kinetic and potential forms. We 

 now proceed to consider in what sense we can assert the constancy in 

 quantity, as well as the permanence of existence of something which 

 appears to us in various forms not directly comparable with each other, 

 i.e. not directly measurable in terms of the same unit. An illustration 

 may assist us here. If a man possesses some bank-notes, some gold, and 

 some silver, he may use them all for purchase, since they are all money ; 

 and he may exchange any one form for one or more of the others. He 

 may change a note for a mixture of gold and silver, and he may change 

 silver for gold or notes. We may, therefore, assert qualitatively that 

 money appears in various forms. But we may go further, and make a 

 quantitative statement. There is a fixed rate of exchange between the 

 various forms, so that he may exchange a 5 note for five sovereigns 

 and each sovereign for twenty shillings. With this fixed rate of 

 exchange, the total amount reckoned in terms of any one form is 

 constant. If, for example, he begins with four 5 notes he may 

 exchange all or any of them for gold or silver, or both, and the total 

 quantity reckoned in terms of any one coin will be constant, and he will 

 possess twenty pounds or four hundred shillings, even though the actual 

 money may be a mixture of paper, gold, or silver. Were there no fixed 

 rate of exchange, this could not be asserted. If, for instance, his money 

 were partly in English, partly in French notes or coins, the amount 

 reckoned in any one form would, of course, vary at different times 

 according to the rate of exchange. The question then arises, does 

 energy undergo its various transformations according to a fixed rate of 

 exchange in each case ? If so, and only on this supposition, the total 

 quantity reckoned in terms of any one form is constant. 



To answer this question, we have, in the first place, to consider how the 

 various kinds of energy are to be measured. Kinetic energy we have 



TUlfi 



defined as measurable by -~ , in terms of a perfectly definite unit, and we 

 know how the mass of any body may be found, at least in theory, so 



* The belief in the identity of energy is no doubt metaphysical, as metaphysical 

 as is our belief in the continued existence of any portion of matter, and its identity 

 under various modifications. As, however, the metaphysical addition somewhat 

 simplifies the form of the statement, and is never likely to lead us wrong in our 

 experimental interpretation, we see no reason to exclude it. Were we to do so we 

 should have to speak of the correlation of the energies, not of the constancy of 

 energy. Instead of describing the conversion of, say, kinetic energy into heat, we 

 should have to say that kinetic energy disappeared, and that at the same time heat 

 energy appeared. 



