NATURAL PHILOSOPHY. 183 



leads to no firmer conviction - t'nan that work cannot be done without pro- 

 ducing an indestructible equivalent of mechanical effect. Various familial- 

 instances of an apparent loss of mechanical effect, as hi the friction, impact, 

 cutting, or bending of solids, were alluded to, but especiaUy that which is 

 presented by a fluid in motion. Although in hammering solids, or in forcing 

 solids to slide against one another, it may have been supposed that the 

 alterations which the solids experience from such processes, constitute the 

 effects mechanically equivalent to the work spent, no such explanation can 

 be contemplated for the case of work spent in agitating a fluid. If water in 

 a basin be stirred round and left revolving, after a few minutes it may be 

 observed to have lost all sensible or otherwise discernible signs of motion. 

 Yet it has not communicated motion to other matter round it ; and it appears 

 as if it has retained no effect whatever from the state of motion in which it 

 had been. It is not tolerable to suppose that its motion can have come to 

 nothing ; and until fourteen years ago confession of ignorance and expectation 

 of light was all that philosophy taught regarding the vast class of natural phe- 

 nomena of which the case alluded to is an example. Mayer, in 1842, and 

 Joule, in 1843, asserted that heat is the equivalent obtained for work spent in 

 agitating a fluid, and both gave good reasons in support of then- assertion. 

 Man}- observations have been cited to prove that heat is not generated by the 

 friction of fluids ; but that heat is generated by the friction of fluids has been 

 established beyond all doubt by the powerful and refined tests applied by 

 Joule in his experimental investigation of the subject. 



An instrument was exhibited, by means of which the temperature of a small 

 quantity of water contained in a shallow circular case provided with vanes in 

 its top and bottom, and violently agitated by a circular disc provided with 

 similar vanes, and made to turn rapidly round, could easily be raised in tem- 

 perature several degrees in a few minutes by the power of a man, and by 

 means of which steam power applied to turn the disc had raised the tempera- 

 ture of the water by 30 degrees in half an hour. The bearings of the shaft, to 

 the end of which the disc was attached, were entirely external ; so that there 

 was no friction of soh'ds under the water, and no way of accounting for the 

 heat developed except by the friction in the fluid itself. 



It was pointed out that the heat thus obtained is noi produced from a source, 

 but is generated ; and that what is called into existence by the work of a man's 

 arm cannot be matter. 



Davy's experiment, in wliich two pieces of ice were melted by rubbing them 

 together in an atmosphere below the freezing point, was referred to as the first 

 completed experimental demonstration of the immateriality of heat, although 

 not so simple a demonstration as Joule's, and although Davy himself gives 

 only defective reasoning to establish the true conclusion which he draws 

 from it. Rumford's inquiry concerning the u Source of the Heat which is 

 excited by Friction " was referred to as only wanting an easy additional expe- 

 riment a comparison of the thermal effects of dissolving (in an acid, for 

 instance), or of burning, the powder obtained by rubbing together solids, with 

 the thermal effects obtained by dissolving or burning an equal weight of the 

 same substance or substances in one mass or in large fragments to prove that 



