SCIENCE IN THE LAST HALF CENTURY. 75 



artificial, hitherto kuown to us— it follows that the speculation that 

 they may be iudeflnitely altered, or that new units may be generated 

 under conditions yet to be discovered, is perfectly legitimate. Theoret- 

 ically, at any rate, the trausmutability of the elements is a verifiable 

 scientific hypothesis; and such inquiries as those which have been set 

 afoot, into the possible dissociative action of the great heat of the 

 sun upon our elements, are not only legitimate, but are likely to yield 

 results which, whether affirmative or negative, will be of great impor- 

 tance. The idea that atoms are absolutely iugenerable and immutable 

 " manufactured articles " stands on the same sort of foundation as the 

 idea that biological species are '-manufactured articles " stood thirty 

 years ago ; and the supposed constancy of the elementary atoms, dur- 

 ing the enormous lapse of time measured by the existence of our uni- 

 verse, is of no more weight against the possibility of change in them, 

 in the infinity of antecedent time, than the constancy of species in 

 Egypt, since the days of Eameses or Obeops, is evidence of their im- 

 mutability during all past epochs of the earth's history. It seems safe 

 to prophesy that the hypothesis of the evolution of the elements from 

 a primitive matter will, in future, play no less a part in the history of 

 science than the atomic hypothesis, which, to begin with, had no greater 

 if so great an empirical foundation. 



It may perhaps occur to the reader that the boasted progress of phys- 

 ical science does not come to much, if our present conceptions of the 

 fundamental nature of matter are expressible in terms employed, more 

 than two thousand years ago, by the old " master of those that know." 

 Such a criticism, however, would involve forgetfulness of the fact that 

 the connotation of these terms, in the mind of the modern, is almost 

 infinitely difiFerent from that which they possessed in the mind of the 

 ancient philosopher. In antiquity, they meant little more than vague 

 speculation; at the present day they indicate definite physical concep- 

 tions, susceptible of mathematical treatment, and giving rise to innu- 

 merable deductions, the value of which can be experimentally tested. 

 The old notions produced little more than floods of dialectics ; the new 

 are powerful aids towards the increase of solid knowledge. 



2. CONSERVATION OF ENERGY. 



Every-day observation shows that of the bodies which compose the 

 material world, some are in motion and some are, or appear to be, at 

 rest. Of the bodies in motion, some, like the sun and stars, exhibit a 

 constant movement, regular in amount and direction, for which no ex- 

 ternal cause appears. Others, as stones and smoke, seem also to move 

 of themselves when external impediments are taken away; but these 

 appear to tend to move in opposite directions, the bodies we call heavy, 

 such as stones, downwards, and the bodies we call light, at least such 

 as smoke and steam, upwards; and as we further notice that the earth 

 below our feet is made up of heavy matter, while the air above our 



