BRITISH SCIENTIFIC ASSOCIATION. 469 



is transmitted through a fine grating, it. is turned aside, or diffracted, according to 

 laws which the wave theory has explained- Mow, Prof. Stokes has shown that, 

 when the incident light is polarized, the plane of vibration of the diffracted ray 

 must differ from that of the incident, the two planes being connected by a very 

 simple relation. It only remained, therefore, for observation to determine whether 

 the planes of polarization of the incident and refracted rays were similarly related 

 or not. The experiment "was undertaken by Prof. Stokes himself, and he has 

 inferred from it that the original hypothesis of Fresnel is the true one. But, as an 

 opposite result has been obtained by M. Holzmann, on repeating the experiment, 

 the question must be regarded as still undetermined. The difference in the 

 experimental results is ascribed by Prof. Stokes to the difference in the nature of 

 the gratings employed by himself and by the German experimentalist, the substance 

 of the diffracting body being supposed to exert an effect upon the polarization of 

 the light,. which is diffracted by it under a great obliquity. I learn from Prof . 

 Stokes that he proposes to resume the experimental inquiry, and to test this 

 supposition by employing gratings-of various substances. If the conjecture should 

 prove to.be well founded, it will greatly complicate the dynamical theoryof li^ht, 

 In the mean time the hypothesis is one of importance in itself, and deserves to be 

 verified or disproved by independent means. 



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" In the whole range of experimental science there is no fact more familiar, or 

 longer known, than the development • of Heat by friction.' The most ignorant 

 savage is acquainted with it, — it was, probably k,nown to the first generation of 

 mankind. Yet, familiar as it is, the science of which it is the germ dates back but 

 a very few years. It was known from the time of Black, that heat disappeared in 

 producing certain changes of state in bodies, and reappeared when the order of 

 those changes was reversed ; and that the amount of heat, thus converted, had a 

 given relation to the effect produced. In one of these changes — namely, evapora- 

 tion, a definite mechanical force is developed, which is again absorbed when the 

 vapour is restored by pressure to the liquid state. It was, therefore, not unnatural 

 to conjecture, that in all cases in which heat is developed by mechanical action, or 

 vice versa, a definite relation would be found to subsist between the amount of the 

 action and that of the heat developed or absorbed. This conjecture was put to the 

 test of experiment by Mayer and Joule, in 1842, and was verified by the result. 

 It was found that heat and mechanical power were mutually convertible ; and that 

 the relation between them was definite, *772 foot-pounds of motive power being ■ 

 equivalent to a unit of heat — that is, to the amount of heat requisite to raise a 

 pound of water through one degree of Fahrenheit. The science of Thermodynam- 

 ics, based upon this fact, and upon a few other obvious facts or self-evident ■ 

 principles, has grown up in the hands of Clausius, Thomson, and Rankine, into 

 large proportions, and is each day making fresh conquests in the region of the 

 unknown. Thus far the science of heat is made to rest wholly upon the facts of 

 experiment, and is independent of any hypothesis respecting the molecular consti- 

 tution of bodies. The dynamical theory of heat, however, has materially aided in 

 establishing true physical conceptions of the nature of heat. The old hypothesis, I 

 of caloric, as a separate substance, was indeed rendered improbable by the experi- 

 ments of Rumford and Davy, and by the reasonings of Young ; but it continued to 

 hold its ground, and is interwoven into the language of science. It is now clearly 

 shown to be self-contradictory ; and to lead to the result that the amount of heat , 



