Part 24

FIG. 71 is the famous ‘Couronne de tasses’. It consists of a series of vessels containing salt water, in which are steeped plates of alternate silver A and zinc Z. The plates are connected by strips of metal a a a. If the first and the last cup be connected by a conductor, a current flows from one to the other.

FIG. 72 is a simple voltaic pile, consisting of alternate disks of silver and zinc, sandwiched between wet strips of leather. The pile is held by glass rods m m m. From the lowermost disk a strip of metal passes to a vessel containing salt water. A current will pass from the uppermost disk to the vessel if the two are connected by a conductor.

FIG. 73 is a similar apparatus with two piles connected by a metal plate c c, and two vessels b b. A current will pass between the two vessels b b if they are joined by a conductor.

§ 5. Discovery of the Nature of the Air.

The seventeenth century saw advances in the knowledge of the air. Boyle (1654, p. 124) had shown by means of his air-pump that air was a material substance and could be weighed. By exhausting the air from a vessel in which an animal had been placed, he showed that it was this material substance and no ether, spirit, or other mysterious entity which supported respiration. Mayow (1668, p. 126) proved that a part only of the air was necessary for life, and later that this same part was removed equally by respiration and combustion (Figs. 74-5). His work was forgotten for a hundred years. The great theorists, Stahl, Boerhaave and Haller, knew him not, and Stahl’s doctrine of phlogiston set back the hands of the clock. No advance was made till the work of Joseph Black (1728-99) which appeared soon after the middle of the eighteenth century.

FIG. 74. A candle is burning and a piece of inflammable material is being ignited in a glass vial by a burning-glass, the mouth of which is under the surface of the water. The air can, if desired, be changed or sampled through the attached tube.

FIG. 75. A mouse confined under a glass cover. The air under this cover communicates with that in the vessel below, and can be cut off more or less completely by means of a more or less porous diaphragm.

Black was a cautious investigator and his success was due to the accuracy of his measurements. He was aware of the fact that chalk, when heated, is transformed into quicklime (equation 1, p. 152), thereby losing its power of effervescing with acids, but gaining the power of absorbing water (equation 2). In modern nomenclature, the changes are:

(1) CaCO{3} = CaO + CO (2) CaO + HO = Ca(OH){2}

The first achievement of Black was to show that in the process of heating the chalk lost weight (equation 1). This was a blow at the phlogiston theory, for it had been supposed that quicklime consisted of chalk plus phlogiston, and that the phlogiston was conveyed to it during the heating. Black now showed that if slaked lime be treated with a mild alkali, such as the carbonate of sodium, it is changed back to the state in which it was before heating, in fact into chalk, while the mild alkali is converted into a caustic alkali. As we now express it:

(3) Ca(OH){2} + Na{2}CO{3} = CaCO{3} + 2NaOH

Black’s triumph consisted essentially in showing that reactions (1) and (3) were indefinitely reversible and that the same amount of CaCO_{3} could always be extracted from (3) as was put into (1). Moreover, he showed that a definite amount of chalk, whether heated into quicklime or not, neutralized an equal weight of acid, the only difference being that the neutralization took place with effervescence and loss of weight if the chalk were unheated, and without effervescence or loss of weight if the chalk were first heated into quicklime. Thus:

(4) Unheated CaCO{3} + 2HCl = CaCl{2} + H{2}O + CO{2} (5) Heated CaO + 2HCl = CaCl{2} + H{2}O

The substance given off by the chalk in (1), absorbed by it in (3), and produced by the reaction (4), he named fixed air. We now call it Carbon dioxide. The conversion of caustic lime into ordinary chalk by exposure, CaO + CO{2} = CaCO{3}, proves that Carbon dioxide is a normal constituent of the atmosphere. Black learned something of its properties, and his work is also of very great importance as the first detailed quantitative study of a chemical reaction and its reversal. The properties of Carbon dioxide were further investigated (1766) by Henry Cavendish (1731-1810).

The next advance in the chemistry of the air was made by the English Unitarian Divine, Joseph Priestley (1733-1804). A series of important observations was made by him in the seventies and eighties of the eighteenth century. He showed that green growing plants would make respired air again respirable, and that they gave off a respirable gas. In 1774 he prepared Oxygen by heating certain oxides, though, still hampered by the phlogiston theory, he failed to recognize the nature of the oxygen he had produced. The conclusions of his striking experiments on blood, which he showed to depend on this same agent for its changes from venous to arterial, were similarly vitiated.

FIG. 76. APPARATUS from Joseph Priestley’s Experiments and Observations on different Kinds of Air, Birmingham, 1774. In the background can be seen an experiment on the effect of combustion on confined air. There are also two cylinders inverted over water in which green plants are growing. In one of them the growing plant has given off a gas (oxygen) which Priestley showed could support both combustion and respiration. In the foreground under a bell-jar are some mice on which Priestley performed respiratory experiments.

The real passage to the modern point of view in our knowledge of the air was made by the brilliant French chemist Antoine Laurent Lavoisier (1743-94). He made an extensive quantitative investigation of the changes during breathing (Fig. 77), burning, and calcination. In the course of these he discovered the true composition of respired air, and showed how both Carbon dioxide and water are normal products of the act of breathing. If clear grasp of the implication of discovery be made the test, Lavoisier must be regarded as the discoverer of Oxygen.

Cavendish (1731-1810) had already discovered the composition of water (1785). Lavoisier concluded that water and Carbon dioxide are produced by the process of oxidation in the lungs, and that it is this oxidization process, and not any innate quality of a mysterious character in the body or in the blood, that is responsible for the bodily heat. Lavoisier introduced much of the chemical nomenclature that we still employ. So far as respiration is concerned, subsequent research has added much to his standpoint. In the purely chemical aspect, however, it has altered little, though we now know that the tissues and not the lungs are the seat of oxidation.

FIG. 77. LAVOISIER in his laboratory making experiments on breathing. To the right Madame Lavoisier sits at a table, taking notes. Lavoisier stands behind, directing. To the left is the subject of the experiment. His face is covered with a mask provided with a valve. He is breathing into the apparatus. An assistant feels his pulse while a second assistant collects the respired air in a bell-jar inverted over a trough.

From a contemporary sketch.

§ 6. Morbid Anatomy becomes a Science.

The main intellectual movement of the seventeenth and eighteenth centuries had been focused, so far as Medicine was concerned, on the manner of working of the animal body, the department that we now term Physiology. It was necessary to obtain clear concepts of the action of the body in health before venturing into discussion of its action in disease. Towards the end of the seventeenth century, an industrious compiler had put together all the then published records of post-mortem examinations up to his time. During the first part of the eighteenth century many practitioners in Physic and in Surgery published isolated cases or groups of cases connected with particular diseases. Boerhaave regularly attended post-mortem examinations (p. 140). No general pathological principles had, however, yet been elicited on a scientific basis. The theories of disease such as those of Boerhaave were perforce still mainly speculative, for there were no extensive records of the correlation of symptoms during life with the appearances of the organs of the body after death, the subject we now call ‘Morbid Anatomy’. This gap was first effectively bridged by Morgagni.

Giovanni Battista Morgagni (1682-1771) was professor at Padua for no less than fifty-six years. During this time he performed an enormous number of post-mortem examinations, and made important contributions to Descriptive Anatomy. In his seventy-ninth year, eleven years before his death, there emerged from his enormous experience his work On the sites and causes of disease. This classical treatise may still be read with profit. Its leading feature is the very careful way in which actual cases are recorded. The life-history of the patient, the history of his disease, the events in connection with his final illness and death, are all recounted with detail and care. The condition of the organs at the post-mortem examination is minutely described and an attempt is made to explain how the symptoms were the result of the lesions. Morgagni is justly said to have introduced the ‘anatomical concept’ into the practice of medicine. This concept is one of the main elements in modern diagnosis, and a modern physician, in reflecting on a case, considers first whether he is able to express the symptoms in terms of lesion. There are many lesions of great importance and frequent occurrence which Morgagni was the first to describe.

The task which Morgagni had undertaken was worthily continued by the Scot, Matthew Baillie (1761-1823), nephew, pupil, and heir of William Hunter (p. 165). Baillie was a successful London practitioner. He followed a new and convenient method in arranging his work according to organs instead of by symptoms, as Morgagni had done. Baillie performed post-mortem examinations on several men of eminence, among them Dr. Johnson, whose lung he describes (see Fig. 78).

The task of naked-eye pathological anatomy, effectively begun by Morgagni, was effectively completed by Karl Rokitansky of Vienna (1804-78). His work (1842-6) was based on an experience extending over 30,000 post-mortems! Though disfigured by a bizarre theory, it left but few gaps for subsequent workers. From now on, the science of Pathology was to be prosecuted in a new spirit and with new instruments. Even in his own day Rokitansky was something of an anachronism, with his pure naked-eye anatomy hardly ever involving experimental evidence on the one hand or the findings of the microscope on the other.

FIG. 78. PART OF THE LUNG OF DR. SAMUEL JOHNSON, from a drawing published by Matthew Baillie. Johnson was a fat, unwieldy man, with a great barrel chest, who suffered for many years from shortness of breath. These are common associations with the pathological condition known as Emphysema, in which the lungs, which are normally of fine spongy texture, become full of abnormally large cavities, so that, as Baillie remarks, they come ‘to resemble the air cells of the lungs of amphibious animals’ (cf. Fig. 45, p. 116). In the figure B represents the external part of the lung and A its cut surface. On the cut surface the large cellular structure can be seen. The very dark points are the orifices of cut branches of the pulmonary vessels.

§ 7. Clinical Methods and Instruments.

The great teachers of the earlier eighteenth century, though better equipped as regards knowledge than their predecessors, had hardly any better means of diagnosis. Pulse-measurers and thermometers such as those of Sanctorius and Galileo (p. 109) had proved impracticable by the bedside. The microscope had not yet entered into Clinical Medicine. Chemical analysis as applied to disease had proved, as yet, of little value.