The 100 Most Influential Scientists of All Time (12 page)

In 1675 Newton brought forth a second paper, an examination of the colour phenomena in thin films, which was identical to most of Book Two as it later appeared in the
Opticks
. The purpose of the paper was to explain the colours of solid bodies by showing how light can be analyzed into its components by reflection as well as refraction. The paper was significant in demonstrating for the first time the existence of periodic optical phenomena. He discovered the concentric coloured rings in the thin film of air between a lens and a flat sheet of glass; the distance between these concentric rings (Newton's rings) depends on the increasing thickness of the film of air.

A second piece which Newton had sent with the paper of 1675 provoked new controversy. Entitled “An Hypothesis Explaining the Properties of Light,” it was in fact a general system of nature. Robert Hooke, who had earlier established himself as an opponent of Newton's ideas, apparently claimed that Newton had stolen its content from him. The issue was quickly controlled, however, by an exchange of formal, excessively polite letters that fail to conceal the complete lack of warmth between the men.

Newton was also engaged in another exchange on his theory of colours with a circle of English Jesuits in Liège, perhaps the most revealing exchange of all. Although their objections were shallow, their contention that his experiments were mistaken lashed him into a fury. The correspondence dragged on until 1678, when a final shriek of rage from Newton, apparently accompanied by a complete nervous breakdown, was followed by silence. For six years he withdrew from intellectual commerce except when others initiated a correspondence, which he always broke off as quickly as possible.

During his time of isolation, Newton, who was always somewhat interested in alchemy, now immersed himself in it. His conception of nature underwent a decisive change. Newton's “Hypothesis of Light” of 1675, with its universal ether, was a standard mechanical system of nature. However, about 1679, Newton abandoned the ether and its invisible mechanisms and began to ascribe the puzzling phenomena—chemical affinities, the generation of heat in chemical reactions, surface tension in fluids, capillary action, the cohesion of bodies, and the like—to attractions and repulsions between particles of matter.

More than 35 years later, in the second English edition of the
Opticks
, Newton accepted an ether again, although it
was an ether that embodied the concept of action at a distance by positing a repulsion between its particles. As he conceived of them, attractions were quantitatively defined, and they offered a bridge to unite the two basic themes of 17th-century science—the mechanical tradition, which had dealt primarily with verbal mechanical imagery, and the Pythagorean tradition, which insisted on the mathematical nature of reality. Newton's reconciliation through the concept of force was his ultimate contribution to science.

In 1684 Newton was at work on the problem of orbital dynamics, and two and a half years later, a short tract he had written, entitled
De Motu
(“On Motion”), had grown into
Philosophiae Naturalis Principia Mathematica
. This work is not only Newton's masterpiece but also the fundamental work for the whole of modern science. Significantly,
De Motu
did not state the law of universal gravitation. For that matter, even though it was a treatise on planetary dynamics, it did not contain any of the three Newtonian laws of motion. Only when revising
De Motu
did Newton embrace the principle of inertia (the first law) and arrive at the second law of motion.

The mechanics of the
Principia
was an exact quantitative description of the motions of visible bodies. It rested on Newton's three laws of motion: (1) that a body remains in its state of rest unless it is compelled to change that state by a force impressed on it; (2) that the change of motion (the change of velocity times the mass of the body) is proportional to the force impressed; (3) that to every action there is an equal and opposite reaction. Using these laws, Newton found that the centripetal force holding the planets in their given orbits about the Sun must decrease with the square of the planets' distances from the Sun.

Newton also compared the distance by which the Moon, in its orbit of known size, is diverted from a tangential path in one second with the distance that a body at the surface of the Earth falls from rest in one second. When the latter distance proved to be 3,600 (60 × 60) times as great as the former, he concluded that one and the same force, governed by a single quantitative law, is operative in all three cases, and from the correlation of the Moon's orbit with the measured acceleration of gravity on the surface of the Earth, he applied the ancient Latin word
gravitas
(literally, “heaviness” or “weight”) to it. The law of universal gravitation, which he also confirmed from such further phenomena as the tides and the orbits of comets, states that every particle of matter in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centres. The
Principia
immediately raised Newton to international prominence.

(b. May 23, 1707, Råshult, Småland, Swed.—d. Jan. 10, 1778, Uppsala)

S
wedish naturalist and explorer Carolus Linnaeus was the first to frame principles for defining natural genera and species of organisms and to create a uniform system for naming them (binomial nomenclature).

In 1735 Linnaeus published
Systema Naturae
(“The System of Nature”), a folio volume of only 11 pages, which presented a hierarchical classification, or taxonomy, of the three kingdoms of nature: stones, plants, and animals. Each kingdom was subdivided into classes, orders, genera, species, and
varieties. This hierarchy of taxonomic ranks replaced traditional systems of biological classification that were based on mutually exclusive divisions, or dichotomies.

In particular, it was the botanical section of
Systema Naturae
that built Linnaeus's scientific reputation. After reading essays on sexual reproduction in plants by Vaillant and by German botanist Rudolph Jacob Camerarius, Linnaeus had become convinced of the idea that all organisms reproduce sexually. As a result, he expected each plant to possess male and female sexual organs (stamens and pistils), or “husbands and wives,” as he also put it. On this basis, he designed a simple system of distinctive characteristics to classify each plant. The number and position of the stamens, or husbands, determined the class to which it belonged, whereas the number and position of pistils, or wives, determined the order. This “sexual system,” as Linnaeus called it, became extremely popular.

In 1736 Linnaeus, then in the Netherlands, published a booklet, the
Fundamenta Botanica
(“The Foundations of Botany”), that framed the principles and rules to be followed in the classification and naming of plants. The year before, Linnaeus was introduced to George Clifford, a local English merchant and banker who had close connections to the Dutch East India Company. Impressed by Linnaeus's knowledge, Clifford offered Linnaeus a position as curator of his botanical garden. Linnaeus accepted the position and used this opportunity to expand certain chapters of the
Fundamenta Botanica
in separate publications: the
Bibliotheca Botanica
(1736; “The Library of Botany”);
Critica Botanica
(1737; “A Critique of Botany”), on botanical nomenclature; and
Classes Plantarum
(1738; “Classes of Plants”). He applied the theoretical framework laid down in these books
in two further publications:
Hortus Cliffortianus
(1737), a catalogue of the species contained in Clifford's collection; and the
Genera Plantarum
(1737; “Genera of Plants”), which modified and updated definitions of plant genera first offered by Joseph Pitton de Tournefort.

Genera Plantarum
was considered by Linnaeus to be his crowning taxonomic achievement. In contrast to earlier attempts by other botanists at generic definition, which proceeded by a set of arbitrary divisions,
Genera Plantarum
presented a system based on what Linnaeus called the “natural characters” of genera—morphological descriptions of all the parts of flower and fruit. In contrast to systems based on arbitrary divisions (including his own sexual system), a system based on natural characters could accommodate the growing number of new species—often possessing different morphological features—pouring into Europe from its oversea trading posts and colonies.

Linnaeus's distinction between artificial and natural classifications of organisms, however, raised the question of the mechanism that allowed organisms to fall into natural hierarchies. He could only answer this question with regard to species: species, according to Linnaeus, were similar in form because they derived from the same parental pair created by God at the beginning of the world. Linnaeus tried to explain the existence of natural genera, orders, or classes within the context of hybridization; however, the question of natural hierarchies would not receive a satisfying answer until English naturalist Charles Darwin explained similarity by common descent in his
Origin of Species
(1859).

In 1738 Linnaeus began a medical practice in Stockholm, Swed., which he maintained until 1742, when he received
the chair in medicine and botany at Uppsala University. Linnaeus built his further career upon the foundations he laid in the Netherlands. Linnaeus used the international contacts to create a network of correspondents that provided him with seeds and specimens from all over the world. He then incorporated this material into the botanical garden at Uppsala, and these acquisitions helped him develop and refine the empirical basis for revised and enlarged editions of his major taxonomic works. During his lifetime he completed 12 editions of the
Systema Naturae
, six editions of the
Genera Plantarum
, two editions of the
Species Plantarum
(“Species of Plants,” which succeeded the
Hortus Cliffortianus
in 1753), and a revised edition of the
Fundamenta Botanica
(which was later renamed the
Philosophia Botanica
[1751; “Philosophy of Botany”]).

Linnaeus's most lasting achievement was the creation of binomial nomenclature, the system of formally classifying and naming organisms according to their genus and species. In contrast to earlier names that were made up of diagnostic phrases, binomial names (or “trivial” names as Linnaeus himself called them) conferred no prejudicial information about the plant species named. Rather, they served as labels by which a species could be universally addressed. This naming system was also implicitly hierarchical, as each species is classified within a genus. The first use of binomial nomenclature by Linnaeus occurred within the context of a small project in which students were asked to identify the plants consumed by different kinds of cattle. In this project, binomial names served as a type of shorthand for field observations. Despite the advantages of this naming system, binomial names were used consistently in print by Linnaeus only after the publication of the
Species Plantarum
(1753).

The rules of nomenclature that Linnaeus put forward in his
Philosophia Botanica
rested on a recognition of the
“law of priority,” the rule stating that the first properly published name of a species or genus takes precedence over all other proposed names. These rules became firmly established in the field of natural history and also formed the backbone of international codes of nomenclature—such as the Strickland Code (1842)—created for the fields of botany and zoology in the mid-19th century. The first edition of the
Species Plantarum
(1753) and the 10th edition of the
Systema Naturae
(1758) are the agreed starting points for botanical and zoological nomenclature, respectively.

Toward the end of his life, Linnaeus became interested in other aspects of the life sciences. Of greatest influence were his physico-theological writings,
Oeconomia Naturae
(1749; “The Economy of Nature”) and
Politiae Naturae
(1760; “The Politics of Nature”). Both works were of great importance to Charles Darwin. His studies of plant hybridization influenced the experimental tradition that led directly to the pea plant experiments of Austrian botanist Gregor Mendel.