This chapter outlines Hopkins’s knowledge of contemporary energy physics as it decisively shapes his distinctive poetry and the metaphysic that undergirds it. The discussion begins with Hopkins’s appreciation of meteorology in his ‘Heraclitean Fire’ sonnet, of the earth’s atmosphere as a vast thermodynamic system. The figure that this poem presents of man as a lonely ‘spark,’ and the pyrotechnics of ‘As kingfishers catch fire,’ ‘The Windhover’ and ‘God’s Grandeur,’ are then glossed through the optical application of the energy concept in spectroscopy. Finally the chapter considers field theory and Clerk Maxwell’s reassessment of the Newtonian principle of force through the energy concept as the distributive principle of stress, tracing Hopkins’s use of this physical concept in his writings on mechanics, nature and most momentously in the definitive formulation of his metaphysic of stress, instress ,and inscape in 1868 and the concurrent advent of his metrical principle of Sprung Rhythm.