Latent Heat and the Invention of the Watt EngineAuthor(s): Donald FlemingSource: Isis, Vol. 43, No. 1 (Apr., 1952), pp. 3-5Published by: The University of Chicago Press on behalf of The History of Science SocietyStable URL: http://www.jstor.org/stable/227128 .

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Latent Heat and the Invention of the

Watt Engine

BY DONALD FLEMING *

M /[ UCH has been written, early and late, on the connection between Joseph Black and James Watt; and few if any writers have entered a radical dissent from the proposition that Watt could not have introduced the separate con-

denser without the aid of Black.' The materials for this view of their relationship lay ready at hand in the oldest layer of traditions concerning the two men. Their common friend, John Robison, in dedicating his edition of Black's chemical lectures to "Dr. Black's most illustrious Pupil," spoke of Watt's "improvements on the steam engine, which you profess to owe to the instructions and information you received from Dr. Black.'' 2 In the text proper Black made the same point:

I have the pleasure of thinking, that the knowledge which we have acquired concerning the nature of elastic vapour, in consequence of my fortunate observation of what happens in its formation and condensation, has contributed, in

no inconsiderable degree, to the public good, by suggesting to my friend Mr. Watt of Birming- ham, then of Glasgow, his improvements on this powerful engine.3

That is, the theory of latent heat was decisive for the invention of Watt's steam engine. Of Robison at least, no reader of the several passages in which he recurs to this point will doubt that he means as far as he can to vindicate the social utility and historical significance of the quiet and selfless academic who shakes the world at one remove. The claims made by Robison and Black have been repeated from generation to genera- tion; and may be found in several works of scrupulous contemporary scholarship.4

The present article proposes to substitute for the traditional view an alternative resting on three propositions: first, that Watt discovered certain empirical consequences of the phenomenon of latent heat, independently of Black; second, that the knowledge of these consequences would have been sufficient for Watt's purpose as a mechanic, without the construction which he learned from Black to put upon them; and third, that the fundamental innovation of a separate condenser did not require a knowledge either of the empirical consequences or of the theoretical doctrine of latent heat.

Substantially these points were made by Watt in I822.5 Though his testimony might be regarded as suspect, it has not been refuted but ignored. The case for his indebted- ness to Black in the matter of latent heat is asserted rather than argued. Before ex- amining the thesis propounded by Watt, we ought to bear in mind that he kept his peace for many years, dealt respectfully with Black and Robison when he spoke out, and gave little evidence of exasperated vanity. He began by drawing up a short list

* Brown University. 1A. Wolf, A History of Science, Technology,

and Philosophy in the Eighteenth Century (Lon- don, I939), pp. 6I8-620, fails to make the con- ventional claim for Black, but says nothing in refutation of it.

2Joseph Black, Lectures on the Elements of Chemistry, John Robison, ed. (Edinburgh, i8O3), I, iii.

3Black, I, I84. 4Cf. H. W. Dickinson and Rhys Jenkins,

James Watt and the Steam Engine (Oxford,

I927), p. 22; H. W. Dickinson, A Short History of the Steam Engine (Cambridge, Eng., I939]), p. I75; Douglas McKie and Niels H. de V. Heathcote, The Discovery of Specific and Latent Heats (London, I935), p. I22; J. R. Partington, A Short History of Chemistry, 2d ed. (London, I948), p. 95.

"Letter to Dr Brewster from Mr Watt," in John Robison, A System of Mechanical Phi- losophy, David Brewster, ed. (Edinburgh, I822),

II, iii-x.

3 Isis, vol. 43, Ap-ril zQ52.

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4 Donald Fleming

of facts known before his time or discovered by others in his time: first, that "steam was condensed by coming into contact with cold bodies"; second, that "water and other liquids boiled in vacuo at very low heats; water below iooO"'; third, that "the capacity . . . of heat . . . was much smaller in mercury and tin than in water"; and fourth, that "evaporation caused the cooling of the evaporating liquid, and bodies in contact with it." 6 To this tabulation he added a list of his own experiments to determine: the capacities for heat of certain substances as compared with water; the bulk of steam compared with that of water; the quantity of water evaporated in a given boiler by a pound of coal; the elasticities of steam at various temperatures above the boiling point of water; the quantity of steam required for each stroke of a Newcomen engine of given proportions; and the quantity of cold water required in every stroke to condense the steam in the cylinder of the Newcomen engine.7 The last experiment taught him that steam could heat six times its own weight of water to 2120 F.

Being struck with this remarkable fact, and not understanding the reason of it, I mentioned it to my friend Dr Black, who then explained to me his doctrine of latent heat, which he had taught for some time before this period, (sum-

mer 1764,) but having myself been occupied with the pursuits of business, if I had heard of it, I had not attended to it, when I thus stumbled upon one of the material facts by which that beautiful theory is supported.8

To my knowledge this statement has never been challenged; and in spite of the lateness of the date, it has the ring of truth about it, with the enthusiastic acknowl- edgment by Watt that the theory of latent heat is entirely the achievement of Black.9 Watt's recollection would indicate that he discovered for himself an unexpected reservoir of heat in steam; and if the existence of this reservoir had been the decisive clue for the improvement of the Newcomen engine, Watt could have done his work without consulting Black. He got from Black an access of understanding; but better machines can sometimes be built on a foundation of brute empirical fact into which no great theoretical insight has yet been afforded. In all good logic, men need not have passed from the Edison effect to the Fleming valve over the bridge of sub-atomic physics.

Yet all of this, in Watt's view, would have been beside the point. In his words:

. . . this theory [of latent heat], though useful in determining the quantity of injection neces- sary where the quantity of water evaporated by the boiler, and used by the cylinder, was known, and in determining, by the quantity and heat of the hot water emitted by Newcomen's engines, the quantity of steam required to work them, did not lead to the improvements I afterwards made in the engine. These improvements proceeded upon the old-established fact, that steam was

condensed by the contact of cold bodies, and the later known one, that water boiled in vacuo at heats below Ioo?, and consequently that a vacuum could not be obtained unless the cylin- der and its contents were cooled every stroke to below that heat.

These, and the degree of knowledge I pos- sessed of the elasticities of steam at various heats, were the principal things it was necessary for me to consider in contriving the new engine.'0

To put the matter in another way, the inconvenience and much of the economic waste of alternately cooling and heating the same cylinder would not have been obviated by the non-existence of latent heat (regarded not as a theory but as a natural phenome- non).

One difficulty remains to be cleared away. If, as a psychological and historical fact, Watt did not pass from the conception of latent heat to that of a separate condenser, how had he come to make the famous experiment which established that steam could

-7Ibid., II, vii-viii. 'Watt, note supplied to Brewster; Robison,

Mechanical Philosophy, II, ii6n. 'Ivor Hart, James Watt and the History of

Steam Power (New York, 1949), p. I62, makes

the point that Watt had uncovered certain em- pirical consequences of the latency of heat, inde- pendently of Black. Cf. Dickinson and Jenkins, Watt, p. 22.

l1Watt, "Letter to Brewster," Robison, Me- chanical Philosophy, II, viii.

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Latent Heat and the Invention of the Watt Engine 5

bring to the boiling point six times its own weight of water? The answer would seem to be that this experiment was merely part of a larger enterprise, namely, an effort at understanding the phenomenon of "back pressure." The down stroke of the piston in a Newcomen engine ought ideally to work against a perfect vacuum induced by con- densing the steam required for the up stroke. The cold injection water used to produce condensation may, however, fail of its purpose and be brought by the heat of steam to the boiling point. The result is back pressure, impeding the down stroke of the piston and producing less of a lift from, say, the mine shaft to which the engine is applied. The problem was greatly complicated by the circumstance, well known to Watt and his contemporaries, that the boiling point of water is drastically lowered by the very process of evacuating the vessel in which the water is contained. Cooling the cylinder of the engine, therefore, to a temperature slightly below 2I20 F. will not suffice to prevent back pressure. In the light of this knowledge, antecedent to the dis- covery of latent heat, a given quantity of steam would possess the power to bring a given quantity of water to a boil at temperatures of less than half the normal boiling- point. This fact, of itself, would require very sharp alternations of cooling and heat- ing the cylinder; and so contribute to that waste of steam which Watt had seized upon as the central attribute of the Newcomen engine. In this engine, the avoidance of back pressure required that steam, on being introduced to effect the up stroke, be diverted in considerable part to re-heating the walls of a cylinder which had just been cooled, and markedly cooled, to produce the down stroke. Now, it is a particular aspect of this matter that owing to the release of latent heat by steam on condensation, an astonishingly large injection of cold water is needed to prevent back pressure. This fact Watt discovered for himself. But on the basis of the records now remaining, one must suppose that he regarded his discovery as merely a further complication of the problem of back pressure. This problem did not arise out of the latency of heat. One may, indeed, imagine that some one, having discovered the latency of heat, and the consequent necessity for very large injections of water into a Newcomen engine, might have concluded that a separate condenser would be a good thing. Yet this is not history, but speculation.

The relationship between the author of the concept of latent heat and the inventor of the separate condenser ought not to be taken as the validating image of the reliance of practice on theory, the dependence of the mechanic Watt on the theorist Black. Another kind of reliance of Watt on Black may perhaps be discerned.

Although [Watt writes] Dr Black's theory of latent heat did not suggest my improvements on the steam-engine, yet the knowledge upon vari- ous subjects which he was pleased to communi-

cate to me, and the correct modes of reasoning, and of making experiments of which he set me the example, certainly conduced very much to facilitate the progress of my inventions. . ..

To say of Black that he inspired the process of reasoned experimentation by which Watt successively improved the steam engine would be a remarkable tribute to a great scientist; and this tribute is authorized by Watt himself.

"1Ibid., II, ix.

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