1. Compare the almost ethereally structured and crystal clear though colourless theory of elasticity in Kirchhoff’s lectures with the crudely realistic account in Vol. 3 of Thomson’s Mathematical and Physical Papers which concerns not ideally elastic bodies but steel, rubber, glue; or with the often childlike naiveté of Maxwell’s language, who in the midst of formulae mentions a really effective method for removing fat stains. 2 Maxwell, Treatise on Electricity and Magnetism 1873, Vol. 1, Sec. 29: Nature of the operator ∇ and ∇2. This was later noticed by others too: Mach, Wien, Sitzungsber. 86 (1882) 8. Cf also Wied. Beibl.
7, 10; Comptes Rendus Ac. de Paris
95, 479. [Editor’s note: Consistency with modern practice (and with Boltzmann’s own in his Vorlesungen über die Principe der Mechanik) would demand ‘∂’ for ‘d’ throughout. For ‘Δ’ it is now more common to use Maxwell’s won ‘∇2.]