Textbook Errors, 59
GUEST AUTHOR David Layton The University Leeds, England
I
The Original Observations of Brownian Motion
M a n y textbooks1 make some brief historical reference to the observations of Robert Brown which led to the discovery of the irregular trembling motion of suspended minute particles associated with his name. I n some cases, the reference is ambiguous with respect to the objects in motion in that it is shted that Brown observed the movement while examining a suspension of pollen grains under the microscope. I n other cases, the reference is plainly erroneous in that it is stated that the pollen grains themselves were observed to execute the motion, although in fact, these are much too large for that. Brown's initial observations, made in the summer of 1827, were on the pollen of Clarkia pulchella. I n diameter these grains are about one hundred microns (10-2cm), approximately two orders of magnitude greater than the diameter of particles which might be expected to show Brownian motion. The title of Brown's paper makes clear that it was not the pollen grains themselves that were seen in motion, and also that the motion was observed with particles other than those from organic sources: "A Brief Account of Microscopical Observations Made in the Months of June, July and August, 1827, on the Particles Contained in the Pollen of Plants; and on the General Existence of Active Molecules in Organic and Inorganic Bodies" ( 1 ) .
The investigation on which Brown was engaged was concerned with the precise nature of the action of pollen on the pistil of a flower. The general notion of the sexuality of flowering plants had been accepted from the end of the seventeenth century, when scientists such as Nehemiah Grew (2) and Camerarius (3) had shown that pollination was essential to the production of seeds. Later, in the hands of Linnaeus, the idea of the sexuality of plants had become the basis for a system of classification. The mechanism of fertilization, however, remained uncertain and had to await a technological innovation, the development of the compound achromatic microscope objective by Selligue and Amici in the 1820's (4).
Suggestions of material suitable for this column and guest columns suitable for publication directly should be sent with as many details as possible, and particularly with references to modern textbooks, to Karol J. Mysels, Department of Chemistry, University of Southern California, Los Angeles, California 90007.
Since the purpose of this column is to prevent the spread and continnation of errors and not the evaluation of individual texts, the source of errors discussed will not he cited. In order to be presented an error must occur in at least two independent recent atandard hooks.
Brown's investigations into the mechanism of fertilization were mainly conducted with a simple microscope equipped with a double convex lens of focal length about inch. From his paper it is clear that the particles executing the irregular trembling motion were from within the pollen grains, presumably cytoplasmic granules. Brown hoped to use the characteristic nonspherical shape of some of these granules for identification purposes in tracing their path within the pistil, but this idea did not prove feasible. However, he did observe that all living plants which he examined yielded particles from within their pollen and an aqueous suspension of the particles always showed the characteristic motion. I n consequence, he was led to question whether the motion ceased with the death of a plant; subsequently specimens dried and preserved for more than a century were found to yield moving particles. The peculiar and general character of this motion suggested it might be employed as a test for the recognition of the male sex organ of a plant; accordingly Brown applied his test to plants such as Moses and Epuisetum-in which, a t that time, the existence of sex organs had not been universally admitted. Again he observed the characteristic motion with particles obtained from the suspected male parts of these plants, both living and dried. At this stage an accidental happening changed the course of the investigation. The chance bruising of the seeds of Eguisetum brought about a great increase in the number of moving particles, and he found that the bruising and suspension of tissues from all other parts of the plants yield particles in motion. Brown then widened his hypothesis, supposing that the particles were peculiar to organic bodies; there was some support for this notion of "active molecules" in the writings of naturalists such as Buffon and Needham (6),and the extension of his observations to a wide range of animal and vegetable tissues lent support to his view. However, Brown was too thorough. He examined suspensions of the products of organic bodies-resins, even, or as he put it, "pit-coal." Finally, from fossils he moved to minerals: from the realm of the organic to that of the indisputably inorganic. In almost every case, a suspension of particles revealed the incessant trembling motion. Even a fragment of the Sphinx yielded particles in Brownian motion. Brown offered no further explanation of the cause of the motion. He attempted to make an estimate of the magnitude of the particles and to determine whether they were of uniform size; his estimate of 1/20,000 inch is surprisingly good, considering the equipment a t his Volume 42, Number 7, July 1965
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disposal. His paper was widely discussed-there is a reference to it, for example, in George Eliot's "Middlemarch" (6)-and 12 months later he felt obliged to publish some "Additional Remarks on Active Molecules" (7). In this short paper, he was at pains to make clear that it was not his belief that the particles were animated and that even if his results (with which he was not fully satisfied) did show that the particles had much the same size and shape, it did not necessarily follow that they were identical in every other respect. He emphasized that the cause of motion was uncertain, but his further experiments enabled him to reject firmly any explanation in terms of the evaporation of liquid, or the mutual attraction and repulsion of particles. In 1829 Faraday, a t a "Friday Evening Discourse" at the Royal Institution, reviewed the evidence on Brownian motion (8). He gave a characteristically balanced and lucid survey of the matter: organic and inorganic particles had been observed to show a trembling irregular motion when suspended in a fluid, and this motion could not be considered as distinctive of vitality. Only when the kinetic molecular theory of matter had become established some 50 years later was a satisfactory explanation of the motion possible (0).
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Brownian motion holds an important place ill any consideration of why we believe in atoms. The propagation of the error that a particle as large as a pollen grain will exhibit an easily observable movement fosters a misleading impression of the scale of the phenomenon. Literature Cited (1) BROWN, R., Phil. Mag., 4 (New Series), 161 (1828). Also in: Edinbur~hPhil. J., 19,358 (1828), and "The Miscellaneous Botanical Worka of Robert Brown," Ray Sooiety Publications, London, 1866, Vol. I, p. 463. ('2) GREW, NEHEMIAH,"The Anatomy of Plants," London, 1682, pp. 171-2. R., "Epistala de sexu plantarum," Tubingen, (3) CAMERAR~S, 1694. Cited by VON SACHB,J., "History of Botany, 1530-1860," translated by GARNSEY, H. E. F. Rev. ed., Clarendon Press, Oxford, 1906, p. 387. (4) LISTER,J. J., Phil. T ~ a m . 1830, , pp. 187-9. (.5) PASTEUR,L., "Memoir on the Organized Corpuscles which J. B., m~ NASA, Exist in the Atmosphere," in CONANT, L. K., "Harvard Case Hktiatories in Experimental Soience," Harvard University Preas, Cambridge, Mass., 1957, Vol. 2, 0.
495.
ELIOT, (;EORGB, ''hl~ddl~rnwrlr,'' Book 11, Ch. XWI. ( 7 ) BROWN, R., P h i l . Mng., 6 , 161 (1829). (8) S w e.g. Phil. 31ng., 5 , 230 (1829); Quorl. J . Sci. 5 ( n . ~ . )363 , I(;,
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(9)DELSAULX, J., "Thermodynamic Origin of the Brownian Motions," in The Monthly Microseopieal J m m l . 18, 1 (1877).
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lournol o f Chemical Education
