MOTION. 33 



Mr. Joule has made a great number of experiments for 

 the purpose of ascertaining what quantity of heat is produced 

 by a given mechanical action. His mode of experimenting 

 is as follows. An apparatus formed of floats or paddles of 

 brass or iron is made to rotate in a bath of water or mercu- 

 ry. The power which gives rise to this rotation is a weight 

 raised like a clock- weight to a certain height ; this by acting 

 during its fall on a spindle and pulley communicates motion 

 to the paddle-wheel, the water or mercury serving as a fric- 

 tion medium and calorimeter ; and the heat is measured by a 

 delicate mercurial thermometer. The results of his experi- 

 ments he considers prove that a fall of 772 Ibs. through a 

 space of one foot is able to raise the temperature of one 

 pound of water through one degree of Fahrenheit's thermom- 

 eter. Mr. Joule's experiments are of extreme delicacy he 

 tabulates to the thousandth part of a degree of Fahrenheit, 

 and a large number of his thermometric data are compre- 

 hended within the limits of a single degree. Other experi- 

 menters have given very different numerical results, but the 

 general opinion seems to be that the numbers given by Mr. 

 Joule are the nearest approximation to the truth yet obtained. 



Hitherto I have taken no distinction as to the physical 

 character of the bodies impinging on each other ; but Nature 

 gives us a remarkable difference in the character or mode of 

 the force eliminated by friction, accordingly as the bodies 

 which impinge are homogeneous or heterogeneous : if the 

 former, heat alone is produced ; if the latter, electricity. 



We find, indeed, instances given by authors, of electricity 

 resulting from the friction of homogeneous bodies ; but, as I 

 stated in my original Lectures, I have not found such facts 

 confirmed by my own experiments, and this conclusion has 

 been corroborated by some experiments of Professor Erman, 

 communicated to the meeting of the British Association in 

 the year 1845, in which he found that no electricity resulted 

 from the friction of perfectly homogeneous substances ; as, 



