In a quiet corner of London, a set of cloudy glass jars collected by Charles Darwin has become the focus of a very modern experiment. Instead of scalpels and pipettes, scientists are pointing precision lasers at these priceless specimens to uncover secrets that have been sealed for nearly two centuries. The goal is simple but high stakes: learn exactly how these animals were preserved without ever breaking the wax seals that protect them.
By probing the liquid around Darwin’s specimens with light rather than opening the containers, researchers are trying to solve a practical conservation problem and a historical mystery at the same time. The work promises to protect a unique scientific legacy while offering a template for how museums might care for millions of fragile specimens in the future.
Darwin’s jars and why they matter so much
Before I can explain the lasers, I need to spell out why these jars are treated almost like crown jewels. Charles Darwin is widely regarded as one of the most influential biologists in history, and the animals he collected helped shape the ideas that became evolutionary theory. Many of those specimens, gathered on voyages and field trips, now sit in the collections of the Natural History Museum, where they are valued not only as scientific material but as physical links to the moment modern biology began. The museum’s own account of Darwin’s life and work underlines how central his collections are to understanding how he developed his theories and how they continue to inform research today, especially in areas like species variation and long term environmental change, which are documented in detail in the museum’s overview of Charles Darwin.
Among those holdings is a series of roughly 200 year old jars containing mammals, reptiles and invertebrates preserved in liquid. The glass is old, the seals are fragile, and in many cases the labels are incomplete or missing, which means curators do not always know what chemicals are inside. That uncertainty is not just a curiosity, it is a risk. If the fluid is evaporating, breaking down or reacting with the specimen, the damage could be irreversible. Yet opening the jars to test the liquid would expose the contents to air, vibration and contamination. For objects that connect directly to Darwin’s own hands and notebooks, that is a line museum staff have been unwilling to cross.
The problem with 200 year old preservatives
The core challenge is that nineteenth century preservation was more craft than standardized protocol. Collectors and preparators used whatever recipes were available or affordable, often mixing alcohols, salts and other chemicals in ways that were never fully documented. Over time, fluids can be topped up, replaced or contaminated, so a jar that started with one solution might now contain a layered chemical history. For Darwin’s jars, that means the liquid surrounding a specimen could be a complex mixture that has changed several times since it was first sealed, and no one can be sure what is in it without testing.
Recent work on these collections has shown that different animal groups were treated in distinct ways, which adds another layer of complexity. Researchers examining the jars have found that mammals and reptiles were often treated with formalin before being stored in alcohol, while invertebrates were more likely to be killed and preserved directly in alcohol or other solutions. That pattern, described in detail in a report on how mammals and other groups were handled, means that even within Darwin’s collection, the chemistry can vary widely from jar to jar. For conservators, guessing wrong about those fluids could mean choosing storage conditions that accelerate decay instead of slowing it.
How lasers read a jar without opening it
To get around that problem, scientists have turned to a technique that treats the jar itself as a kind of optical puzzle. Instead of sampling the liquid directly, they shine a laser through the glass and measure how the light scatters after it passes through the fluid. The method, known as Spatially Offset Raman Spectroscopy, or Spatially Offset Raman Spe for short, detects the unique vibrational fingerprints of different molecules even when they are hidden behind a barrier. In practical terms, it lets researchers identify chemicals inside an opaque or sealed container by analyzing the returning light, rather than the liquid itself, which is why it has become so attractive for heritage science.
Earlier this year, a study demonstrated that portable laser spectroscopy units using Spatially Offset Raman Spe could be brought directly into museum stores and used on historic jars in situ. The researchers showed that the technique could distinguish between common preservatives like ethanol, methanol and formalin without disturbing the container, and they argued that the same approach could be scaled up to survey entire collections. Their findings, described in a detailed Jan Phys Post, make the case that this is not just a clever trick for a handful of Darwin relics but a practical tool for managing millions of specimens worldwide that sit in similar jars.
What the new scans revealed about Darwin’s jars
When researchers applied this laser method to Darwin’s own specimens, they were able to do something curators had wanted for years: map the chemical contents of the jars without lifting a single lid. By comparing the spectral signatures from the laser with reference data, the team correctly identified the preservation fluids in jars that had long been mysteries. In some cases, they confirmed that the liquid was still a relatively pure alcohol solution, while in others they detected evidence of formalin treatment or more complex mixtures that had changed over time. According to a summary of the project, the study revealed how preservation fluids and storage methods have shifted over the centuries and, crucially, did so while keeping the specimens intact, a result highlighted in a report noting that scientists have successfully Darwin’s original material without compromising its integrity.
The work did more than satisfy curiosity about old recipes. By documenting which jars contain formalin treated mammals and reptiles and which hold invertebrates in simpler alcohol solutions, the team created a chemical map that can guide future conservation decisions. If a particular fluid is known to degrade glass or labels, staff can prioritize those jars for environmental control or repackaging, while jars with more stable solutions can be monitored less intensively. A detailed account of the project explains how the scanning technique lets the people who care for these collections understand not just what is in each jar but how those contents have evolved, a point underscored in coverage of how analysing Darwin specimens without opening them can reshape collection management.
Why this matters far beyond Darwin’s cabinet
As striking as it is to see lasers trained on Darwin’s jars, the broader implications are even more significant. Natural history museums around the world hold millions of fluid preserved specimens, many of them collected in the same era and under similarly undocumented conditions. For those institutions, the ability to identify unknown preservatives without opening containers could transform how they plan storage, safety protocols and research access. One report on the new technique stresses that it offers a way to survey large collections efficiently, flagging jars that contain hazardous or unstable chemicals and helping staff decide where to invest limited conservation resources, a benefit that is especially clear in the description of how a groundbreaking laser technique can determine unknown preservatives.
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*This article was researched with the help of AI, with human editors creating the final content.
