Contents of jars collected by Charles Darwin revealed in new scans

Almost two centuries after Charles Darwin’s pioneering voyage on the HMS Beagle, his specimens are still inspiring groundbreaking new science.

Many of the specimens collected on the voyage are now looked after by us, from the finches and mockingbirds that helped inspire Darwin’s thoughts on evolution to the fossils of an extinct giant ground sloth.

While some of these specimens were stored as dried skins and bones, other were preserved in fluid and join the 23 million other specimens in our spirit collection.

Over the centuries, the animals these jars contain have been well studied, but there is still a lot we don’t know about the actual fluids they were preserved in. This is an issue as opening the jars without knowing what’s inside could accidentally expose scientists to toxic chemicals or cause a specimen to degrade.

But a new approach developed in our collections offers a solution. By shining lasers through the jars, scientists are able to use the way that the light scatters to decipher the chemical makeup of the fluids inside.

This technique is able to accurately identify the contents of nearly 80% of the jars studied and partially identify 95% of the samples. Chelsea McKibbin, our Senior Conservator and a co-author of the research, says that refining the process further could help museum curators to protect scientifically valuable and sometimes irreplaceable specimens.

“The specimens used in this study represent 200 years of scientific collecting, including Darwin’s original samples that helped shape our understanding of evolution,” Chelsea says.

“Being able to analyse their preservation without opening their containers means we can maintain the integrity of these historical materials while ensuring they remain available for future generations to learn from and be inspired by.”

The findings of the study were published in the journal ACS Omega.

How are wet collections preserved?

Jars of preserved specimens, known as wet or spirit collections, are a key part of natural history museums.

They preserve whole animals and plants as they were in life by using fluids to keep them as fresh as possible by protecting them from microbes that would cause them to rot. But how well the specimens are preserved largely depends on which fluids they’re kept in.

Over the history of natural history collections, different people have tried different fluids with varying results. Some of the earliest known wet specimens were prepared in the 1600s by Dutchman Frederik Ruysch, who used a variety of solutions including a mixture of water, alcohol and spices such as clove, pepper and cardamom.

Though the spices were quickly dropped, concentrated alcohols remained a popular choice because of their ability to kill microbes. Historic manuals recommend scientists use rum or whisky, while modern curators prefer pure methanol or ethanol.

Formaldehyde solutions, such as formalin, became common from the late 1800s. These preserve the tissues of dead organisms by introducing new chemical bonds to hold them together.

But the same properties that make these chemicals good at preserving dead organisms can also make them dangerous to us. These include compounds containing toxic elements like mercury, or historic preservatives such as picric acid which can become explosive.

Spotting these potential hazards, however, is challenging because the fluids used for preservation in the past were often a matter of personal preference and rarely recorded. Even when the composition of a fluid is written down, mistakes and contamination can mean that labels are no longer accurate.

Co-author Dr Sara Mosca, from the Science and Technology Facilities Council’s Central Laser Facility, says that the method the team have developed helps to protect the specimens and scientists alike.

“Until now, understanding what preservation fluid is in each jar meant opening them, which risks evaporation, contamination, and exposing specimens to environmental damage,” explains Sara. “This technique allows us to monitor and care for these invaluable specimens without compromising their integrity.”

Studying jars without opening them

The scanning process uses a technique known as spatially offset Raman spectroscopy. This shines a laser through the jar and into the liquid, which is measured as it emerges on the other side. The change in energy of the light can then be used to figure out which molecules are present in the fluid.

While this technique had previously been achieved in a laboratory, this new research is the first time it has been carried out in a museum collection. The team used a handheld device to investigate the jars and then compared the results to the chemical signatures of known preservative mixtures.

In total, the researchers investigated 46 specimens, including jars containing fishes collected by Darwin, mammals such as the coati and a variety of clams and jellyfish. Their analysis successfully revealed the fluids surrounding 36 of the specimens, while some of the chemicals in a further seven were also identified.

They found that different organisms tended to have been preserved differently. Mammals and reptiles had often been preserved using formalin and then stored in alcohol, while invertebrates were often kept in formalin or mixtures containing additives like propylene glycol.

Wren Montgomery, one of our research technicians who co-authored the study, says that knowing these details will help curators to know what’s normal for a specimen and help them to intervene before serious problems set in.

“This work demonstrates the Natural History Museum’s commitment to transforming the study of natural history and caring for our collections,” Wren adds. “Analysing the storage conditions of specimens, such as those collected by Darwin nearly 200 years ago, without disturbing them preserves them so that future researchers can understand more about our planet.”