"From Dalton in 1803 to Maxwell in 1866 the atomic picture of matter had progressively taken on shape and detail: by Maxwell's time it... lay at the heart of the classical system. ...all matter was composed of ponderable atoms and, with the 'death' of caloric, the division between corporeals and incorporeals became absolutely sharp. The known incorporeal agencies—radiant heat, light, magnetisim and electricity—had to be dealt with in terms of other concepts."