60 Henry Riddell on 



discovery with regard to radiant heat, that its rays were reflected 

 and refracted in the same manner as those of light, was held to 

 be a strong argnment in its favour. It must be remembered 

 that the emission theory of light was still in force, and that 

 Newton's explanation of refraction was absolutely consistent with 

 the then known facts, the constant relation between the sines. 

 of the incident and emergent ray. You remember Newton's 

 calculation as to the force with which one grain of glass attracts 

 the light which it refracts, that it is not less than the force with 

 which a wire would be stretched by a hundred millions of millions 

 ot pounds of matter hanging by it. In one of his notes to Black's 

 lectures Robison says — " This observation of the constant ratio 

 of the sines of incidence and refraction, is the strongest argument 

 that has yet been obtained for the. materiality of heat. For this 

 law of motion is competent only to a material particle, having 

 mobility and inertia, and acted on by transverse accelerating 

 forces, in the same manner that a cannon ball is acted on by 

 gravity and by the force of projection." It is now universally 

 accepted that heat is a form of energy, but what energy is 

 independent of mxatter is a difficult conception. Kadiant heat 

 comes from the sun exactly as light, and it would seem clear that 

 a vibration which our theories tell us exists in the ether of space, 

 must be transmitted to and taken up by material bodies. The 

 possible relations between the ether and matter are still to 

 explain. 



As has already been said there are no other great original 

 investigations of Black's to be mentioned, but much good, if 

 rather routine work was done. The most outstanding example 

 was the analysis of water from a hot mineral spring, in which the 

 methods employed were very novel and greatly in advance of the 

 practice of the time. He was often invited to advise on problems 

 of industrial Chemistry, and offered many acute suggestions. 



It has been quite impossible to go into Black's work in 

 detail, and it is fully time to say something of his family and 

 friends. 



