Above: biologist Wilhelm J. Schmidt’s (1884–1974) schematic diagram of the molecular structure of protoplasm, initially drawn for the 1939 centennial celebrations of Schleiden and Schwann’s cell theory. My historical research is centered on this image — how it was made, where it came from, the physical and chemical science behind it, and how contemporary biologists understood and used it.

You can download and read my articles on my PhilPapers profile. Please email me at dan@dan-liu.net if you have trouble obtaining any of my publications.

In 2021–2024 I am the Principal Investigator (Eigene Stelle) for the three-year, DFG-funded project ““Living Matter Under the Microscope: Protoplasm Theory, ‘wissenschaftliche Mikroskopie,’ and the Molecularization of Cell Structure, 1840–1940” (DFG Project 463389772). This project explores the history of cell theory and theories of “living matter”—theories that the cell’s material constitution is what makes it alive—from 1840 to 1940. During this historical period, three critical developments in the history of cell biology took place. First, the 1838–39 cell theory of Matthias Schleiden and Theodor Schwann was augmented in the 1860s by Ernst Brücke and Max Schultze’s protoplasm theory, which stated that the so-called “protoplasm,” or the interior contents of the cell, was the material bearer of life itself. Second, through the work of Ernst Abbe and other German microscopists, the microscope was understood to have an absolute physical limit to its resolution, the so-called Abbe diffraction limit of half the wavelength of the imaging light. Third, in this period physicists and chemists made dramatic discoveries in atomic and molecular theory, which meant that biologists claiming to follow the strictures of physics and chemistry had to repeatedly revise their theories of material or molecular structure as they applied it to cells, protoplasm, and other biological materials. Taken together, these three developments created a new domain of biological research that sought to understand the structure and composition of protoplasm, and which recast German Zellforschung into a materialist science and a materials science. My project therefore asks two questions. First: what impact did the development and later decline of protoplasm theory have on the development of the life sciences as a whole? And second: how did biologists study protoplasm? The central premise of this project is that biologists in this period understood developments in both protoplasm theory and the physical sciences more broadly through their methodological engagement with the microscope and microtechnique—the suite of staining, fixation, and preparation techniques that made “wissenschaftliche Mikroskopie” possible. By studying how the technology of microscopy coevolved with protoplasm theory, this project takes a historiographically innovative approach that treats a conceptual history and the history of instrumentation on equal terms.



with Mathias Grote, Lisa Onaga, Angela N. H. Creager, Soraya de Chadarevian, Gina Surita, and Sarah E. Tracy, “The Molecular Vista: Current Perspectives on Molecules and Life in the Twentieth Century,” History and Philosophy of the Life Sciences 43 no. 1 (March 2021), doi:10.1007/s40656-020-00364-5. Peer reviewed, open access

This essay considers how scholarly approaches to the development of molecular biology have too often narrowed the historical aperture to genes, overlooking the ways in which other objects and processes contributed to the molecularization of life. From structural and dynamic studies of biomolecules to cellular membranes and organelles to metabolism and nutrition, new work by historians, philosophers, and STS scholars of the life sciences has revitalized older issues, such as the relationship of life to matter, or of physicochemical inquiries to biology. This scholarship points to a novel molecular vista that opens up a pluralist view of molecularizations in the twentieth century and considers their relevance to current science.


“Scaling from Weather to Climate,” in Weathering, ed. Christoph Holzhey and Arnd Wedemeyer, Cultural Inquiry 17 (ICI Berlin Press, 2020), 93–118.


“The Artificial Cell, the Semipermeable Membrane, and the Life That Never Was, 1864-1901,” Historical Studies in the Natural Sciences 49 no. 5 (November 2019): 504–555, doi:10.1525/hsns.2019.49.5.504. Peer reviewed.

Since the early nineteenth century a membrane or wall has been central to the cell’s identity as the elementary unit of life. Yet the literally and metaphorically marginal status of the cell membrane made it the site of clashes over the definition of life and the proper way to study it. In this article I show how the modern cell membrane was conceived of by analogy to the first “artificial cell,” invented in 1864 by the chemist Moritz Traube (1826–1894), and reimagined by the plant physiologist Wilhelm Pfeffer (1845–1920) as a precision osmometer. Pfeffer’s artificial cell osmometer became the conceptual and empirical basis for the law of dilute solutions in physical chemistry, but his use of an artificial analogue to theorize the existence of the plasma membrane as distinct from the cell wall prompted debate over whether biology ought to be more closely unified with the physical sciences, or whether it must remain independent as the science of life. By examining how the histories of plant physiology and physical chemistry intertwined through the artificial cell, I argue that modern biology relocated vitality from protoplasmic living matter to nonliving chemical substances—or, in broader cultural terms, that the disenchantment of life was accompanied by the (re)enchantment of ordinary matter.


“Heads and Tails: Molecular Imagination and the Lipid Bilayer, 1917–1941,” in Visions of Cell Biology: Reflections Inspired by Cowdry’s General Cytology, ed. Karl Matlin, Jane Maienschein, and Manfred Laubichler (Chicago: University of Chicago Press, 2018), 209–45. Peer reviewed.

Today, the lipid bilayer structure is nearly ubiquitous, taken for granted in even the most rudimentary introductions to cell biology. Yet the image of the lipid bilayer, built out of with lipids with heads and tails, went from having obscure origins deep in colloid chemical theory in 1924 to being “obvious to any competent physical chemist” by 1935. This chapter examines how this schematic, strictly heuristic explanation of the idea of molecular orientation was developed within colloid physical chemistry, and how the image was transformed into a reflection of the reality and agency of lipid molecules in the biological microworld. Whereas in physical and colloid chemistry these images considered secondary to instrumental measurement and mathematical modeling of surface phenomena, in biology the manipulable image of the lipid on paper became an essential tool for the molecularization of the cell.


“The Cell and Protoplasm as Container, Object, and Substance, 1835–1861” Journal of the History of Biology 50, no. 4 (November 2017): 889–925, doi:10.1007/s10739-016-9460-9. Peer reviewed. Winner of the 2020 Everett Mendelsohn Prize for the best article in JHB in the preceding three years, doi:10.1007/s10739-020-09593-7

This article revisits the development of the protoplasm concept as it originally arose from critiques of the cell theory, and examines how the term protoplasm transformed from a botanical term of art in the 1840s to the so-called “living substance” and “the physical basis of life” two decades later. I show that there were two major shifts in biological materialism that needed to occur before protoplasm theory could be elevated to have equal status with cell theory in the nineteenth century. First, I argue that biologists had to accept that life could inhere in matter alone, regardless of form. Second, I argue that in the 1840s, ideas of what formless, biological matter was capable of dramatically changed: going from a “coagulation paradigm” (Pickstone, 1973) that had existed since Theophrastus, to a more robust conception of matter that was itself capable of movement and self-maintenance. In addition to revisiting Schleiden and Schwann’s original writings on cell theory, this article looks especially closely at Hugo von Mohl’s definition of the protoplasm concept in 1846, how it differed from his primordial utricle theory of cell structure two years earlier. This article draws on Lakoff and Johnson’s theory of ‘‘ontological metaphors’’ to show that the cell, primordial utricle, and protoplasm can be understood as material container, object, and substance, and that these overlapping distinctions help explain the chaotic and confusing early history of cell theory.

Essay review, “This Is the Synthetic Biology That Is,” for Studies in History and Philosophy of Biological and Biomedical Sciences vol. 63 (June 2017): 89–93, doi:10.1016/j.shpsc.2017.03.002.

Review of: Sophia Roosth, Synthetic: How Life Got Made (University of Chicago Press, 2017); and Andrew S. Balmer, Katie Bulpin, and Susan Molyneux-Hodgson, Synthetic Biology: A Sociology of Changing Practices (Palgrave Macmillan, 2016).

with Amanda DeMarco, “A Flâneur, But So What?: Franz Hessel and Objectivity in Weimar Berlin,” Los Angeles Review of Books Blog, June 26, 2017. https://blog.lareviewofbooks.org/essays/flaneur-franz-hessel-objectivity-weimar-berlin/

When he dons the mantle of the flâneur, though Franz Hessel may be disrupting certain aspects of Weimar culture, equally so is he affirming others, in particular, its interest in dispassionate, objective observation of the city around him.

with Amanda DeMarco, “Paul Scheerbart and the Art of Science,” Los Angeles Review of Books, March 18, 2017. https://lareviewofbooks.org/article/paul-scheerbart-and-the-art-of-science/

The visionary science fiction writer Paul Scheerbart (1863–1915) was obsessed by the role creativity plays in scientific discovery, as well as creative applications of scientific ideas. We review several recent English translations of Scheerbart’s obsessive, avant garde fiction, and explore its connections both to popular writings by the biologist Ernst Haeckel and fin-de-siècle debates about the scientific imagination.


“Visions of Life and Matter: Protoplasm, Scientific Microscopy, and the Origins of Molecular Biology, 1839–1941” (Ph.D. dissertation, University of Wisconsin-Madison, June 2016).