A History of Science, vol 3 by Henry Smith Williams (important of reading books txt) 📖

the mechanical equivalent of heat then became the main corner-stone of the greater law of the conservation of energy.

 

But while this citation is fresh in mind, we must turn our attention with all haste to a country across the Channel—to Denmark, in short—and learn that even as Joule experimented with the transformation of heat, a philosopher of Copenhagen, Colding by name, had hit upon the same idea, and carried it far towards a demonstration. And then, without pausing, we must shift yet again, this time to Germany, and consider the work of three other men, who independently were on the track of the same truth, and two of whom, it must be admitted, reached it earlier than either Joule or Colding, if neither brought it to quite so clear a demonstration. The names of these three Germans are Mohr, Mayer, and Helmholtz. Their share in establishing the great doctrine of conservation must now claim our attention.

 

As to Karl Friedrich Mohr, it may be said that his statement of the doctrine preceded that of any of his fellows, yet that otherwise it was perhaps least important.

In 1837 this thoughtful German had grasped the main truth, and given it expression in an article published in the Zeitschrift fur Physik, etc. But the article attracted no attention whatever, even from Mohr’s own countrymen. Still, Mohr’s title to rank as one who independently conceived the great truth, and perhaps conceived it before any other man in the world saw it as clearly, even though he did not demonstrate its validity, is not to be disputed.

 

It was just five years later, in 1842, that Dr. Julius Robert Mayer, practising physician in the little German town of Heilbronn, published a paper in Liebig’s Annalen on “The Forces of Inorganic Nature,” in which not merely the mechanical theory of heat, but the entire doctrine of the conservation of energy, is explicitly if briefly stated. Two years earlier Dr. Mayer, while surgeon to a Dutch India vessel cruising in the tropics, had observed that the venous blood of a patient seemed redder than venous blood usually is observed to be in temperate climates. He pondered over this seemingly insignificant fact, and at last reached the conclusion that the cause must be the lesser amount of oxidation required to keep up the body temperature in the tropics. Led by this reflection to consider the body as a machine dependent on outside forces for its capacity to act, he passed on into a novel realm of thought, which brought him at last to independent discovery of the mechanical theory of heat, and to the first full and comprehensive appreciation of the great law of conservation. Blood-letting, the modern physician holds, was a practice of very doubtful benefit, as a rule, to the subject; but once, at least, it led to marvellous results. No straw is go small that it may not point the receptive mind of genius to new and wonderful truths.

 

MAYER’S PAPER OF 1842

 

The paper in which Mayer first gave expression to his revolutionary ideas bore the title of “The Forces of Inorganic Nature,” and was published in 1842. It is one of the gems of scientific literature, and fortunately it is not too long to be quoted in its entirety.

Seldom if ever was a great revolutionary doctrine expounded in briefer compass:

 

“What are we to understand by ‘forces’? and how are different forces related to each other? The term force conveys for the most part the idea of something unknown, unsearchable, and hypothetical; while the term matter, on the other hand, implies the possession, by the object in question, of such definite properties as weight and extension. An attempt, therefore, to render the idea of force equally exact with that of matter is one which should be welcomed by all those who desire to have their views of nature clear and unencumbered by hypothesis.

 

“Forces are causes; and accordingly we may make full application in relation to them of the principle causa aequat effectum. If the cause c has the effect e, then c = e; if, in its turn, e is the cause of a second effect of f, we have e = f, and so on: c = e = f … = c.

In a series of causes and effects, a term or a part of a term can never, as is apparent from the nature of an equation, become equal to nothing. This first property of all causes we call their indestructibility.

 

“If the given cause c has produced an effect e equal to itself, it has in that very act ceased to be—c has become e. If, after the production of e, c still remained in the whole or in part, there must be still further effects corresponding to this remaining cause: the total effect of c would thus be > e, which would be contrary to the supposition c = e. Accordingly, since c becomes e, and e becomes f, etc., we must regard these various magnitudes as different forms under which one and the same object makes its appearance. This capability of assuming various forms is the second essential property of all causes. Taking both properties together, we may say, causes an INDESTRUCTIBLE

quantitatively, and quantitatively CONVERTIBLE objects.

 

“There occur in nature two causes which apparently never pass one into the other,” said Mayer. “The first class consists of such causes as possess the properties of weight and impenetrability. These are kinds of matter. The other class is composed of causes which are wanting in the properties just mentioned—

namely, forces, called also imponderables, from the negative property that has been indicated. Forces are therefore INDESTRUCTIBLE, CONVERTIBLE, IMPONDERABLE OBJECTS.

 

“As an example of causes and effects, take matter: explosive gas, H + O, and water, HO, are related to each other as cause and effect; therefore H + O =

HO. But if H + O becomes HO, heat, cal., makes its appearance as well as water; this heat must likewise have a cause, x, and we have therefore H + O + X =

HO + cal. It might be asked, however, whether H + O

is really = HO, and x = cal., and not perhaps H + O =

cal., and x = HO, whence the above equation could equally be deduced; and so in many other cases. The phlogistic chemists recognized the equation between cal. and x, or phlogiston as they called it, and in so doing made a great step in advance; but they involved themselves again in a system of mistakes by putting x in place of O. In this way they obtained H =

HO + x.

 

“Chemistry teaches us that matter, as a cause, has matter for its effect; but we may say with equal justification that to force as a cause corresponds force as effect. Since c = e, and e = c, it is natural to call one term of an equation a force, and the other an effect of force, or phenomenon, and to attach different notions to the expression force and phenomenon. In brief, then, if the cause is matter, the effect is matter; if the cause is a force, the effect is also a force.

 

“The cause that brings about the raising of a weight is a force. The effect of the raised weight is, therefore, also a force; or, expressed in a more general form, SEPARATION IN SPACE OF PONDERABLE OBJECTS IS A FORCE; and since this force causes the fall of bodies, we call it FALLING FORCE. Falling force and fall, or, still more generally, falling force and motion, are forces related to each other as cause and effect—forces convertible into each other—two different forms of one and the same object. For example, a weight resting on the ground is not a force: it is neither the cause of motion nor of the lifting of another weight. It becomes so, however, in proportion as it is raised above the ground.

The cause—that is, the distance between a weight and the earth, and the effect, or the quantity of motion produced, bear to each other, as shown by mechanics, a constant relation.

 

‘Gravity being regarded as the cause of the falling of bodies, a gravitating force is spoken of; and thus the ideas of PROPERTY and of FORCE are confounded with each other. Precisely that which is the essential attribute of every force—that is, the UNION of indestructibility with convertibility—is wanting in every property: between a property and a force, between gravity and motion, it is therefore impossible to establish the equation required for a rightly conceived causal relation.

If gravity be called a force, a cause is supposed which produces effects without itself diminishing, and incorrect conceptions of the causal connections of things are thereby fostered. In order that a body may fall, it is just as necessary that it be lifted up as that it should be heavy or possess gravity. The fall of bodies, therefore, ought not to be ascribed to their gravity alone. The problem of mechanics is to develop the equations which subsist between falling force and motion, motion and falling force, and between different motions. Here is a case in point: The magnitude of the falling force v is directly proportional (the earth’s radius being assumed—oo) to the magnitude of the mass m, and the height d, to which it is raised—that is, v = md. If the height d = l, to which the mass m is raised, is transformed into the final velocity c = l of this mass, we have also v = mc; but from the known relations existing between d and c, it results that, for other values of d or of c, the measure of the force v is mc squared; accordingly v = md = mcsquared. The law of the conservation of vis viva is thus found to be based on the general law of the indestructibility of causes.

 

“In many cases we see motion cease without having caused another motion or the lifting of a weight. But a force once in existence cannot be annihilated—it can only change its form. And the question therefore arises, what other forms is force, which we have become acquainted with as falling force and motion, capable of assuming? Experience alone can lead us to a conclusion on this point. That we may experiment to advantage, we must select implements which, besides causing a real cessation of motion, are as little as possible altered by the objects to be examined. For example, if we rub together two metal plates, we see motion disappear, and heat, on the other hand, make its appearance, and there remains to be determined only whether MOTION is the cause of heat. In order to reach a decision on this point, we must discuss the question whether, in the numberless cases in which the expenditure of motion is accompanied by the appearance of heat, the motion has not some other effect than the production of heat, and the heat some other cause than the motion.

 

“A serious attempt to ascertain the effects of ceasing motion has never been made. Without wishing to exclude a priori the hypothesis which it may be possible to establish, therefore, we observe only that, as a rule, this effect cannot be supposed to be an alteration in the state of aggregation of the moved (that is, rubbing, etc.) bodies. If we assume that a certain quantity of motion v is expended in the conversion of a rubbing substance m into n, we must then have m + v - n, and n = m + v; and when n is reconverted into m, v must appear again in some form or other.

 

By the friction of two metallic plates continued for a very long time,