The term ‘magnetic field’ was introduced by Faraday in 1845, and subsequently adopted by Thomson and Maxwell, whose usage clearly echoed Faraday's. Thomson first used the expression ‘field of feree’ in a letter to Faraday in 1849, following their discussion of the nature of magnetism; and Maxwell first referred to a ‘magnetic field’ in a letter to Thomson in 1854, in the context of a discussion of Faraday's ideas. Maxwell gave the term ‘field’ its first clear definition, in consonance with previous usage, in his paper ‘A dynamical theory of the electromagnetic field’ (1865); there he stated, ‘The theory I propose may therefore be called a theory of the Electromagnetic Field, because it has to do with the space in the neighbourhood of the electric or magnetic bodies’. The concept of a field was to be contrasted with an action-at-a-distance theory of electric action; that is, the mediation of forces by the agency of the contiguous elements of the field existing in the space between separated electrified bodies was to be distinguished from the action of forces operating directly between electrified bodies across finite distances of space.

The inclusive breadth of Maxwell's definition of the field makes it apparent that the physical status of the field was not defined uniquely. In a field theory the forces between bodies were mediated by some property of the ambient space or field.

In style and content, the physical theory of 1850 shows a marked contrast to that prevalent in 1800. By 1850 the limits and internal cohesion of the science of ‘physics’ were clearly articulated, and the subject had achieved a new and well-defined conceptual content and unity. By 1850 some of the main themes of nineteenth-century physics had been formulated: the unification of physical phenomena within a single explanatory framework, the primacy of mechanical explanation as an explanatory programme, the mathematisation of physical phenomena and the role of mathematical analogy as a guide to the formulation of physical theories, and the enunciation of the principle of energy conservation as a universal, unifying law. The emergence of these broad and unifying themes contrasts with the disunity in physical theory in 1800.

The general disjunction in eighteenth-century physical theory can be illustrated by a contrast between Newton's Philosophiae naturalis principia mathematica [Mathematical principles of natural philosophy] (1687) and his Opticks (1704). In the Principia Newton offered the paradigm of a mathematical science of ‘rational mechanics’, and though he expressed the hope that all physical phenomena could be subsumed under analogous mathematical methods (illustrating his intentions by a mathematical treatment of optical refraction), in the Opticks he based his treatment of the problems of optics and chemistry on an experimental methodology and a speculative theoretical structure, an atomistic physics that became bloated in later editions to include a variety of explanatory agents, forces, active principles, and the ether.

In the ensuing section the focus shifts via the slave trade from Africa to the West Indies and the southern United States, while it continues to remain riveted on black resistance to yellow fever and malaria.

As mentioned previously, medical science has not acknowledged that blacks were gifted with innate yellow fever protection relative to whites. Yet that resistance is discernible in the vast epidemiologic laboratory of history, which Chapter II explores. It is not, however, always clearly discernible, because acquired immunity to yellow fever sometimes camouflages innate protection. Therefore special (perhaps even excessive) care has been taken in disentangling the two, so that the statistical evidence upon which much of the chapter rests, evidence generated in abundance by yellow fever's repeated assaults on cities of the West Indies and the American South leaves no doubt of the blacks' superior ability to host the disease and survive.

By contrast much of the reason why blacks have historically proved resistant to malaria is known to medicine; yet, perhaps paradoxically, it is much more difficult to spy this resistance at work than to detect blacks' resistance to yellow fever.

Yellow fever's appearances were specific and limited epidemic events, which means that differential morbidity and mortality by race are easily linked to those appearances. But malaria fevers worked constantly, year in and year out, quietly killing not only directly, but indirectly as well, by weakening the victim, leaving him susceptible to the fatal effects of intercurrent diseases.

European physicians on the coast attempted early to understand why their compatriots failed so miserably to survive those “fevers” to which the black seemed impervious. Unfortunately, their devotion to epidemiologic thought, aged some one-and-a-half millennia, yielded the explanation that the West African sun unbalanced humors in European bodies. Worse, their efforts to correct humoral imbalances in feverstricken victims by draining the body of blood and bile must have contributed heavily to the mounting European death toll. Doubtless many would have survived these dehydrating illnesses had not physicians joined pathogens in depleting body fluids.

Yet not all medical knowledge derived from the ancients was without value. In his classic scrutiny of Airs, Waters, and Places, Hippocrates had speculated that “bad air” might carry disease. Although established in antiquity, this notion had never firmly embedded itself in European thought; continental physicians preferred to rely on Galen. After yellow fever and malaria managed their respective migrations to the Americas and embattled physicians proved helpless against the pair on both sides of the Atlantic, however, the hypothesis began to receive some serious attention. Galen's long reign was at last threatened by changes in medical thought quite literally in the air, and a new empiricism began to permeate the medical profession.

The great English physician Thomas Sydenham (1624–89) was an important figure in this movement urging colleagues to abandon the theoretic for empiric observation, while the migration of Europeans to the Americas under the stimulus of colonization and tropical wars provided a new generation of physicians with bountiful opportunities to observe the mayhem wrought by these diseases firsthand.

This section dealing with black-related disease susceptibilities is the core of the book. One argument that permeates its six chapters should be a familiar one by now, namely that black disease susceptibilities, like immunities, did much to inculcate in physicians, slaveholders, and some unmeasurable portion of the literate public the idea that, from a scientific standpoint, blacks were indeed a different breed of man. At this point, however, a few words about the evidence that supports this argument seem appropriate.

Unquestionably, a good deal of the medical literature concerned with the health differences of black and white was produced in the sectional heat that led to the Civil War-produced, in the words of Todd Savitt, by physicians who … “were writing for an audience who wished to hear that blacks were distinct from whites. This was, after all, a part of the proslavery argument” and incidentally a part we treat in Section IV.

Yet, just because some physicians who wrote about black health differences were committed to the southern cause, it does not necessarily follow that they themselves were not convinced of that distinctiveness, nor that they failed to convince others. Moreover, there was also much antebellum medical literature on the black written long before the Civil War that contains no trace of political taint. Finally in this connection, notions of black-white medical differences hardly originated in North America. Rather, as we have already seen, physicians in Africa and the West Indies were aware of and writing about racial dissimilarities in disease susceptibilities long before white Southerners, physicians included, felt compelled to wield a pen in slavery's defense.

Antebellum physicians – like old gossips-delighted in passing along novel observations, and some of their juiciest tidbits dealt with the peculiar habits of slaves. One custom they tittered about was the “universal practice among them of covering their head and faces, during sleep, with a blanket, or any kind of covering they can get hold of,” even when it meant leaving the feet exposed. Another was the slaves' tendency to bundle up for sleep despite the temperature, while still another was their tendency to sleep head first close to roaring fires regardless of the season. Astounded visitors joined physicians in reporting that even “in the hottest days in summer they are never without fires in their huts,” and according to at least one observer, the bondsmen's fascination with fire had a substantial impact on plantation architecture. “The kitchens,” he remarked “are always at the distance of several yards from the principal dwelling. This is done as well to guard against the house negroes through carelessness setting the house on fire, for they generally sit over it half the night … over a large fire in the summer, when I could scarcely endure the excessive heat of the night in open air.”

One of the many controversies touched off by Robert Fogel and Stanley Engerman's “cliometric” examination of the peculiar institution concerns slave nutrition. The cliometricians portrayed the slave diet as not only substantial calorically but as actually exceeding “modern (1964) recommended daily levels of the chief nutrients.” This portrayal stands in sharp contrast to a more accepted view that has also found “cliometric” support. Richard Sutch, after reworking the Fogel and Engerman calculations, concluded that Time on the Cross claimed too much, that the caloric intake of slaves was “neither excessive nor generous,” and the diet, far from being balanced, was dangerously deficient in many of the chief nutrients.

This clash of cliometricians over nutrients and nutriments has had the heuristic effect of introducing students of the South to such novel preoccupations as livestock slaughter rates and conversion ratios, the kinds of sweet potatoes consumed, and even the proper way to cook cowpeas. As in so many areas of the study of slavery, “cliometrics” (in this case cliometric calorie counting) has brought a new sophistication to an old argument, and we are satisfied with the Fogel and Engerman findings regarding the kinds and quantities of foods generally available to slaves. Indeed, we have accepted these findings, modified them with assumptions suggested by antebellum literature and black genetic circumstances and then used the amounts of foods in question as the basis for a chemical analysis of the slave diet with an eye to ascertaining its quality.

Slave medicine was a strange melange of medical dogma and superstition, pragmatic practice and compulsive conjecturing, insights and ignorance – all of which was aimed at the preservation and perpetuation of those lives in which planters had invested so heavily.

Nonetheless, all slaveholders did not employ physicians regularly, and some, particularly masters of marginal units, seldom, if ever, engaged their services either for slaves or for their own families no matter how grave the illness. Even on these numerous benighted outposts, however, the antebellum physician made regular bedside appearances if only by proxy. Sometimes he was present in the form of a cure laboriously jotted down, or vaguely recalled, obtained from a neighbor who had once consulted a doctor, or perhaps a remedy passed along in the family-materia medica which had become part of the public domain.

He was also frequently represented by one of a multitude of do-ityourself home medical guides authored by enterprising physicians which in themselves are a fascinating mixture of the erudite and the erroneous.

The shores of West Africa were the point of embarkation for the bulk of the 10 million or so blacks who unwillingly left Africa to labor on the plantations and in the cities and mines of the New World. Among these unhappy voyagers were the progenitors of today's North American black population. Yet although the black diaspora ceased long ago the West African homeland continues to exert an enormous physical influence on the descendants of the original migrants, both in terms of outward appearance and biochemical anomalies that have dictated a different black disease experience from the North American white.

West African climate, disease, and relative isolation are the three factors chiefly responsible for this influence. These factors combined and recombined over millennia to mold and shape the inhabitants. They were sheltered by the Sahara desert against easy access from North Africa and its succession of empires and emperors, while turbulent seas and contrary wind patterns discouraged intrusion from the South Atlantic. This is not to say that no caravans crossed the desert to West Africa or that no ships visited West African shores prior to the Portuguese explorations of the fifteenth century.

But contact with the outside world was limited even though regular trade routes did criss-cross West Africa and penetrate into the heart of the continent.

The dawn of the twentieth century saw the black death rate standing at 30.2 per 1,000 as opposed to a white rate of 17.3. Seven decades later (1972) the black death rate had been reduced to 9.4 per 1,000 and the white to 9.3 per l,000. Superficially it would seem that national selfcongratulation is in order. Not only has the black death rate been cut by two-thirds, but today it is virtually the same as the white. Tuberculosis, which was killing turn-of-the century blacks at a rate well over 450 per 100,000 (three times the white rate), had by the years 1929–31 declined substantially in virulence removing only 199 for every 100,000 live population (yet still selecting blacks over whites at a 3:1 rate). Today the disease kills a mere 8 blacks per 100,000 population, although parenthetically the racial preference of the disease has increased. As of 1972 the black tuberculosis mortality rate was almost five times that of whites.