ELECTRO-PHYSIOLOGY. 299 



same time, the frog will at that moment be observed to contract and tlie needle 

 of the galvanometer to deviate ; in order that this metallic contact between the 

 two springs may continue for a very short, yet still measureable, space of time, 

 or, in other words, that the current may occupy a very minute but ascertained 

 interval, it will suffice to have a large wooden wheel, and to fix the strip of brass 

 on the rim of this wheel. Let us suppose the wheel to be three metres in 

 diameter, to revolve in a third of a second, and the strip of brass to be one milli- 

 metre in extent. It is evident, the wheel having this velocity, that when 

 the two springs which are in contact with its rim, touch the plate of brass, there 

 will be a closure of the circuit, and the current will pass for an interval of 

 time, which, under these conditions, would be 9-5,70 of a second. You will first 

 observe that while the frog contracts as if the circuit had remained closed for 

 gome time, the needle of the galvanometer does not deviate, which evinces that 

 the frog is an instrument more sensitive to electric discharges and to sudden 

 and extremely evanescent variations of electricity than the needle of the galva- 

 nometer, which has a degree of inertia, and requires that the action of the current 

 should endure for a certain time in order to move it. In the next place, if the 

 passage of the current be repeated until a sensible quantity of zinc becomes 

 oxidized in the battery, or the circuit be kept closed for a given time, we may 

 succeed in measuring with exactness what is the quantity of zinc oxidized dur- 

 ing that short closure of the circuit, that is, what quantity of electricity by pass- 

 ing in that minute interval produces a certain contraction, which may be calcu- 

 lated, as I said in the first lecture, from the product of the weight raised by the 

 muscle and the height to which it is raised. 



We have thus deduced from experiment, and with much exactness, two quan- 

 tities between which a close connexion must exist — the connexion, in general 

 terms, of cause and effect. These two numbers are the amount of zinc oxidized 

 in the battery or the quantity of electricity produced, and the mechanical labor 

 represented by the muscular contraction excited by that electricity. In order to 

 explain more clearly the connexion here spoken of, I must bring to your notice 

 very briefly one of the finest inductions of modern physics, the mechanical theory 

 of heat. 



Even a slightly attentive observation of physical facts, which are constantly 

 reproduced before us, suffices to vindicate a principle which rational mechanics 

 had demonstrated a priori : the principle of living forces. In a word, neither 

 matter nor force is created in nature, aud consequently neither matter nor force 

 is destroyed. *In every machine which is in motion there is always a motive 

 force and a resistance to be overcome, and, if we would not admit the absurdity 

 of perpetual motion, we must necessarily conclude that the motive labor, so to 

 epeak, and the labor of resistance must be equal in the same interval of time. 

 When, by means of a lever with unequal arms, we see a small weight attached 

 to tbe extremity of the long arm produce an equilibrium with a nmch greater 

 weight at the extremity of the shorter arm, we might for a moment deem it an 

 illusion ; but no sooner is the lever put in motion than we see the principle 

 verified of which we have spoken, namely, that the product of the arm of the 

 lever by the weight will be equal on one side aud on the other, which implies 

 that the greater weight will traverse a space proportionabiy less than that 

 traversed by the smaller weight, exactly in the ratio of the weights. 



Although these truths were demonstrated in rational mechanics, they seemed 

 to meet with some contradiction in physics and in experimental mechanics. In 

 the collision of bodies, as when a body falls from a certain height to the earth 

 or moves in the midst of water, in all cases of pressure, of resistance, of attrition, 

 we were content to say that the forces were dissipated in vibratory movements, 

 diffused themselves gradually into the great masses, were in effect annihilated. 

 On the contrary, all these cases are only examples of transformation of forces — 

 that is to say, of the living force which is transformed into caloric, or vice versa, 



