In this, and all cases requiring scientific precision, it is better to use the units of the metrical system than our clumsy English standards; but it may be sufficient for the ordinary reader to take the metre, which is about 39·37 inches, as practically a yard, and the kilogramme, which is 15,432 English grains, as practically equal to 2 lbs. This is sufficient to show the much greater energy of the invisible forces which act at minute distances, than that of gravity and other forces which do appreciable mechanical work, the energy of a weight falling from a height of more than 1,300 feet being only sufficient to heat its own weight by 1°. This proof of the convertibility of work into heat gives much greater precision to our ideas respecting the real nature of heat and its kindred molecular and atomic energies. Heat is clearly not a material substance, for a body does not gain weight by becoming hotter. In the case of all ponderable matter down to the atoms, which are only of the size of cricket-balls compared to that of the earth, any combination which adds matter adds weight, and the weight of the product exactly equals the sum of the weights of the separate factors which have united to form it. Thus, if iron is burnt in oxygen gas, the product, oxide of iron or rust, weighs more than the original iron by just as much as the weight of the oxygen which has been consumed. But heat, light, and electricity add nothing to the weight of a body when they are added to it, and take nothing away when they are subtracted. The inference is unavoidable that heat, like light, is not ponderable matter, but an energy transmitted by waves of the imponderable medium known as ether. This is confirmed by finding that when a ray from the sun is analysed by passing through a refracting prism, one part of the spectrum shows light of various colours, while another gives heat. The hottest part of the spectrum lies in the red and beyond it, showing that the heat-waves are longer, and their oscillations slower, than those of light. Heat-waves also may be made to interfere, and to become polarised, in a manner analogous to the phenomena exhibited by those of light. There can be no doubt, therefore, that heat, like light, is an energy or mode of motion, transmitted by waves of an imponderable ether, and that it acts on the molecules and atoms of matter by the accumulated successive impulses of those waves on the molecules and atoms which are floating in it, or rather which are revolving in it, in definite groups and fixed orbits, like miniature solar systems or starry universes. We can now see how heat performs work, and why work can be transformed into it.