The study of the mechanical and physical conditions in the deep interior of the stars is undertaken primarily in the hope that an understanding of the internal mechanism will throw light on the external phenomena accessible to observation. More than fifty years have gone by since the general mode of attack was first developed; and the scope of the inquiry has grown so that it now involves much of the recently won knowledge of atoms and radiation, and makes evident the ties which unite pure physics with astrophysics. It would be hard to say whether the star or the electron is the hero of our epic.

The reader will judge for himself whether solid progress has been made. He may, like Shakespeare, take a view less optimistic than my own—

but I hope he will not be so unkind as to continue the quotation—

Re-reading this work I find passages where I have been betrayed into too confident assertion. It is only too true that the most patent clues may mislead, and observational tests of the rough kind here possible sometimes flatter to deceive. But the subject is a fair field for the struggle to gain knowledge by scientific reasoning; and, win or lose, we find the joy of contest.

146. Results reached in the present Chapter have been used in anticipation from § 89 onwards. We must therefore return and take up the problem of the absorption coefficient as it presented itself in § 88. At that stage we were occupied with our first astronomical result of importance, viz. that for the series of giant stars from type M to type A the opacity is nearly constant although the internal temperature increases twelvefold between the beginning and end of the series. This suggested (but, as we now see, wrongly) that the opacity might tend to a constant value at high temperatures and so be the same for all stars. Actually, however, the constancy of the opacity was a statistical result applying to groups of stars presumed to be of the same average mass, and there was no test whether the constancy continued for stars of a different mass.

The radiation in the main interior of a star consists of X rays, and comparison is invited with measurements of absorption of X rays made in the laboratory. In § 105 we have found the absorption coefficient at the centre of Capella to be 49 c.g.s. units. This is of the general order of magnitude of the measured coefficients of most elements for hard X rays; for example, it agrees with the coefficient for iron for wave-length about 0·8 Å. It must, however, be noted that the radiation at the centre of Capella is of much greater wave-length, the maximum intensity being at 3·2 Å.

CEPHEID VARIABLES

123. Although variable stars of the Cepheid type show a periodic change of radial velocity it is improbable that they are binary systems. The theory which now seems most plausible attributes their variation to the pulsation of a single star; and accordingly the varying radial velocity measures the approach and recession of the surface presented towards the observer as the star swells and contracts. If this explanation is correct we have an opportunity of extending the study of the internal state of a star from static to disturbed conditions.

The leading facts about these variables ascertained by observational study are as follows—

About 170 galactic Cepheids are known with periods ranging from a few hours to about 50 days; so-called “orbits” have been determined for 20 of these from measurements of radial velocity. In addition large numbers of Cepheids have been found in some globular clusters; among these periods less than 12 hours are especially prevalent. Cepheids have also been found in the Andromeda nebula.

Relatively few periods are between 0·7 and 3 days, so that the Cepheids may be subdivided into two groups with periods above and below this gap.

The light-range rarely exceeds 1m·2 visual; the photographic range is greater than the visual. The spectral type changes during the period, corresponding to a higher temperature at maximum than at minimum.

The light-curve and the velocity-curve are closely similar; the correspondence is the more marked because both curves are usually unsymmetrical.

1. At first sight it would seem that the deep interior of the sun and stars is less accessible to scientific investigation than any other region of the universe. Our telescopes may probe farther and farther into the depths of space; but how can we ever obtain certain knowledge of that which is hidden behind substantial barriers? What appliance can pierce through the outer layers of a star and test the conditions within?

The problem does not appear so hopeless when misleading metaphor is discarded. It is not our task actively to “probe”; we learn what we do learn by awaiting and interpreting the messages dispatched to us by the objects of nature. And the interior of a star is not wholly cut off from such communication. A gravitational field emanates from it, which substantial barriers cannot appreciably modify; further, radiant energy from the hot interior after many deflections and transformations manages to struggle to the surface and begin its journey across space. From these two clues alone a chain of deduction can start, which is perhaps the more trustworthy because it is only possible to employ in it the most universal rules of nature—the conservation of energy and momentum, the laws of chance and averages, the second law of thermodynamics, the fundamental properties of the atom, and so on. There is no more essential uncertainty in the knowledge so reached than there is in most scientific inferences.

The two lines of investigation which are brought together in the present theory of the equilibrium of a star originate in two classical papers—

1. J. Homer Lane. On the Theoretical Temperature of the Sun. Amer. Journ. of Sci. and Arts, Series 2, Vol. 4, p. 57 (1870).

2. K. Schwarzschild. Ueber das Gleichgewicht der Sonnenatmosphäre. Göttingen Nachrichten, 1906, p. 41.

The latter paper develops the theory of radiative equilibrium in a form appropriate to the outer layers of a star.

Investigations up to the year 1907 are brought together in

1. 3. R. Emden. Gaskugeln: Anwendungen der Mechanischen Wärmetheorie. (B. G. Teubner, Leipzig and Berlin, 1907.)

which contains important developments by Emden himself. The most relevant portions are here summarised in §§ 54–63. Schwarzschild's work, which had newly appeared, is described by Emden, p. 330, but the book is in the main a study of convective equilibrium.

Two further references of historic interest may be added—

1. 4. R. A. Sampson. On the Rotation and Mechanical State of the Sun. Memoirs R.A.S. 51, p. 123 (1894).

2. 5. I. Bialobjesky. Sur l'Équilibre Thermodynamique d'une SphÈre Gazeuse Libre. Bull. Acad. Sci. Cracovie, May, 1913.

The first definitely postulates radiative equilibrium rather than convective equilibrium in the sun's interior. The second takes account of radiation pressure and demonstrates its importance in investigations of the internal equilibrium of a star.

For other early papers the references in Emden's Gaskugeln should be consulted.

My own investigations originated in an attempt to discuss a problem of Cepheid variation.

BICHAT AND THE ECOLE PRATIQUE

Had Xavier Bichat been chosen, in the early summer of 1799, for the official teaching position in what was now designated the Ecole de médecine, he would have been suddenly thrust into a pivotal position in the school's new program. The death of Honore Fragonard (1732–99) had vacated the post of chef des travaux anatomiques in the faculty's recently annexed Ecole pratique. The physicians of the School had welcomed the subsidiary institution “into their bosom” in 1795, when the new, more ecumenical, academic regime began in earnest: Physicians and surgeons now toiled under a single institutional roof.

Since that first year Fragonard had been chef, directing a staff of prosecteurs that included André Duméril (1774–1860) and Guillaume Dupuytren (1777–1835). Both were capable young surgeons and accomplished anatomists. Later in the Napoleonic period, Dupuytren would become known as a dominant figure in his own right (Chapter 3). Though anxious to gain entry to the inner sanctum of the Ecole staff, Bichat no doubt recognized the likelihood that one of the two prosectors already in place, even though they were both considerably his juniors, would ultimately be named to succeed Fragonard, competitive concours or not. At the end of June, Bichat dropped out of the running.

I have tried in this account to provide evidence for the proposition that ideas spread unevenly. Medical traditions are influenced by chance and by context. How, for example, can one account for the differential reception of Bichatian pathology in France and England? And how did pathological anatomy become entrenched in different degrees and ways in different parts of Napoleonic Europe? Various explanations suggest themselves. Perhaps, for example, the science of pathology took different turns on opposite shores of the channel because the material biological reality itself differed between London and Paris.

According to this explanation, patterns of disease would offer sufficiently disparate stimulus to the medical imagination to create ultimately quite different explanatory frameworks. Tissue pathology, for example, might have emerged where there was an isolated superabundance of disease of the serous and mucous membranes, a state of affairs known to exist in the Paris of 1800. Or the new laboratory discipline of toxicology and its sibling, experimental physiology, might have emerged where there was a conspicuous excess of poisoning and newly discovered poisonous materials. This sort of material argument, though attractive, fails finally to persuade. There is too little evidence to suggest that patterns of morbidity and mortality varied significantly between Paris and the urban and military concentrations in Britain or elsewhere.

In 1835 Adolph Muehry, a Hanover physician and surgeon, followed time-honored custom with a Grand Tour of British and European medical institutions. In the next year he published his notes on the relative state of medicine in his native Germany alongside that of England and France. Surveying the development of pathological anatomy in England, Muehry identified a succession of medical men who had been most active in furthering the traditions of English morbid anatomy. He singled out the work of Matthew Baillie (1761–1823) and John Hunter (1728–1793) in the late eighteenth century. He noted that they had been followed by one individual, John Richard Farre (1775–1862), and that Farre in turn had been more lately succeeded by a new group ascendant in the 1830s. Among the latter group Muehry numbered Richard Bright, Thomas Hodgkin, and Robert Carswell.

Muehry's three groups, each separated from the next by half a generation, provide appropriate guideposts around which to locate the changing fortunes of English pathological anatomy in the early nineteenth century. Only the first and third of these “generations,” that of Hunter and Baillie and that of Bright, Hodgkin, and Carswell, are in any detail known to the twentieth century historian. But if, over the critical half-century 1790–1840, each group of physicians was disposed toward certain intellectual predilections, social groups, and professional sensibilities, who was this “missing link”? Who was John Farre?

John Richard Farre had practiced medicine in his native West Indies before seeking to establish a London medical career.

Was the veneration of Bichat a matter of mere expediency, occasioned by the need to move a few bones? How did this young outsider's career come to assume, after his premature death, an almost totemic value? Much of the answer lies in the structure of the French medical community in the postrevolutionary period. In life, as we have seen on the one hand, Bichat was never a central figure in that community. But he fashioned a career, on the other hand, that embodied key features of an emerging professional culture. For decades to come, the image his life and work conjured up was a tightly woven tapestry of the medical and surgical concerns knit together by the revolution. Bichat's memory bound them together still further. The full extent of how it did so is the central concern of this chapter and Chapters 2 through 4.

MEDICAL COMMUNITIES: THE PROFESSIONAL STAGE

Xavier Bichat arrived in Paris at an explosive time in the history of French institutions.