356 Sir W. Suow Harris on a General Law of 



fine wirejon, fig. 5, passing through the thermometer-ball; au 

 impulsive movement thus becomes mechanically communicated, 

 as it were, to the general mass, which, pressing by its elasticity 

 on the surface of the fluid in the reservoir beneath, causes the 

 fluid to ascend along the scale of the instrument. The current 

 of discharge in its momentary passage through the wire un- 

 doubtedly excites in it a greater or less degree of heat. Still the 

 efi"ect is very evanescent, and, as it appears to me, there is not 

 the least ground for concluding that the entire mass of the air 

 in the thermometer-ball experiences an elevation of temperature ; 

 to effect this, some short, but still very sensible portion of time 

 would be requisite, but little or no time elapses. No sooner has 

 the electrical discharge passed through the wire, than the wire 

 appears instantly to recover its original temperature. The ther- 

 mometer-fluid, which at the instant rapidly ascends the scale, as 

 rapidly and immediately descends, and not unfrequently sinks 

 below the zero-point from which it started, a phaenomenon quite 

 inconsistent with the notion that the temperature of the mass of 

 the air in the ball had been permanently elevated, and which if 

 so elevated would necessarily demand some time to cool down 

 again to its previous point. How the doctrines of specific heat 

 can be well applied to such an action as this is not by any means 

 clear. Heat is certainly not added to the mass of air in the ball, 

 or even to the wire in the ordinary way. The heat excited in 

 the wire appears to be the result of a momentary mechanical 

 action, just as we render a nail red-hot by a few blows of a 

 hammer ; but however this may be, we can scarcely venture, in 

 the present imperfect state of our knowledge of the causes of 

 heat and electricity, to apply abstract theoretical formulae to the 

 indications of such an instrument, the precise value of which, as 

 a measure of a certain species of electrical force, we can only 

 arrive at empirically by experiment ] and after all we must not 

 take the instrument for more than it is worth. 



25. Experimentally, however, we find a marked accordance 

 between the degrees of movement of the fluid along the scale, 

 and well-established laws of electricity. Take, for example, the 

 well-established law expressed in the formula F = Q^, anticipated 

 by Cuthbertsou, and first verified by myself in 1830 (14), al- 

 though not referred to by ]\I. De la Rive at p. 146, tom. ii. of 

 his recent work ; here we observe that if twice the quantity of 

 electricity be accumulated and discharged through the wire, the 

 fluid ascends to four times the height ; three times the quantity 

 causes it to reach nine times the altitude. Then, again, take 

 the relative conducting powers of various metals. If, as is 

 pretty well ascertained, we take the heating cff'ect in the simple 

 inverse ratio of the conducting power, we find equal wires of 



