21 6 THE POPULAR SCIENCE MONTHLY. 



ciate with the action of oxygen, then of course unknown) — a gen- 

 eralization useful in itself, and accompanied by an explanation 

 which was not in its origin objectionable. Let us consider, in 

 illustration, any familiar instance of oxidation, and try to look 

 first for what was reasonable in the eighteenth-century views of 

 the cause of such phenomena. A piece of dry wood has in it the 

 power of giving out heat and light when set on fire ; but after it 

 is consumed there is left of it only inert ashes, which can give 

 neither. Something, then, has left the wood in the process of be- 

 coming ashes ; virtue has gone out of it, or, as we should say, its 

 potential energy has gone. 



This is, so far, an important observation, extending over a wide 

 range of phenomena, and, if it had presented itself to the prede- 

 cessors of Newton, it would probably have been allied to the 

 vibratory theories, and become proportionately fruitful. But to 

 his disciples, and to chemists and others who, without being per- 

 haps disciples, were, like all then, more or less consciously influ- 

 enced by the materiality of the corpuscular theory, it appeared 

 that this also was a material emanation, that this energy was an 

 actual ingredient of the wood — a crudeness of conception which 

 seems most strange to us, but it is not, perhaps, unaccountable in 

 view of the then current thought. 



I have said that the progress of science is not so much that of 

 an army as of a crowd of searchers, and that a call in a false 

 direction may be responded to, not by one only, but by the whole 

 body. In illustration, observe that during the greater part of the 

 entire eighteenth century this doctrine was adopted by almost 

 every chemist and by most physicists. It had quite as general an 

 acceptance among scientific men then as the kinetic theory of 

 gases, for instance, has now, and, as far as time is any test of 

 truth, it was tested more severely than the kinetic theory has yet 

 been ; for it was not only the lamp and guide of chemists, and, to 

 a great extent, of physicists also, but it remained the time-honored 

 and highest generalization of chemico-physical science for over 

 half a century, and it was accepted not so much as a conditional 

 hypothesis as a final guide and a conquest for truth which should 

 endure always. And now where is it ? Dissipated so utterly 

 from men's minds that, to the unprofessional part of even an 

 educated audience like this, " phlogiston," once a name to conjure 

 with, has become an unmeaning sound. 



There" is no need to insist on the application of the obvious 

 moral to hypotheses of our own day. I have tried to recall for a 

 moment all that " phlogiston " meant a little more than a hundred 

 years ago, partly because it seems to me that, though a chemical 

 conception, physics is not wholly blameless for it, but chiefly be- 

 cause before it quitted the world it appears to have returned to 



