[66] For a careful examination of the several theories of heat consult Dr. Young’s Course of Lectures on Natural Philosophy, &c., Lecture 52, _On the Measures and the Nature of Heat_; also Powell’s very excellent _Reports on Radiant Heat_--Reports of the British Association, 1832, 1840. The transcendental view which the immaterial theory leads to, cannot be better exemplified than by the following quotation from that inexplicable dream of a talented man, _Elements of Physiophilosophy_, by Lorenz Oken, M.D. (translated for the Ray Society, by Alfred Tulk):-- “Heat is not matter itself any more than light is; but it is only the act of motion in the primary matter. In heat, as well as in light, there certainly resides a material substratum; yet, this substratum does not give out heat and light; but the _motion_ only of the substratum gives out heat, and the _tension_ only of the substratum light. There is no body of heat; nitrogen is the body of heat, just as oxygen may be called the body of fire. Heat is real space; into it all forms have been resolved, as all materiality has been resolved into gravity, and all activity, all polarity, into light. Heat is the universal form, consequently the want of form.” [67] Mémoires de la Société Physique, &c., de Genève, tom. ii. art. 2. [68] This curious phenomenon was first observed by Mr. Trevelyan, whose _Notice regarding some Experiments on the Vibration of Heated Metals_ will be found in the Transactions of the Royal Society of Edinburgh, vol. xii., 1837. In a Memoir in the same volume, entitled _Experimental Researches regarding certain vibrations which take place between metallic masses having different temperatures_, Professor Forbes draws the following conclusions:-- 1. “The vibrations never take place between substances of the same nature. 2. “Both substances must be metallic. (This is now proved not to be necessary.) 3. “The vibrations take place with an intensity proportional (within certain limits) to the difference of the conducting powers of the metals for heat or electricity; the metal having the least conducting power being necessarily the coldest. 4. “The time of contact of two points of the metals must be longer than that of the intermediate portions. 5. “The impulse is received by a distinct and separate process at each contact of the bar and block, and in no case is the metallic connection of the bearing points in the bar, or those of the block, in any way essential. 6. “The intensity of the vibration is (under certain exceptions) proportional to the difference of temperature of the metals.”--Transactions of the Royal Society of Edinburgh, vol. xii. [69] The Bakerian Lecture. _On certain Phenomena of Voltaic Ignition, and the Decomposition of Water into its Constituent Gases by Heat_: by W. R. Grove, Esq.--Philosophical Transactions, 1847. Part 1. [70] Davy’s _Researches on Flame_. Works, vol. vi.--Philosophical Transactions for 1817.