One thing that scientists and historians of science understand instinctively, but I think is underappreciated elsewhere, is that scientists’ assumptions shape not only how we interpret experimental results, but also how we design our experiments.
The ways we think about things, what we understand our experimental subjects and variables to be, certain assumptions we make about them, are baked into experimental design. If we use mice as a model organism, we assume that mice basically do what we think they do, and any weirdness we observe is therefore probably mostly due to the experiment.* If we are working with light, we assume it behaves mostly like a transverse electromagnetic wave – since, quantum weirdness aside, that is what we understand it to be.**
It’s always difficult to wrap our heads around our own understandings and assumptions, since we tend to see them as facts of life, the way the world is. This is where history of science, looking at people who lived in a different world and thought in very different ways, can be very useful in helping us to understand ourselves. So I’d like to use this post to discuss an experiment I came across in my own research, and talk about how the assumptions of an 18th century surgeon shaped his experiments.
James Lind was a ship’s doctor, a surgeon in the British Royal Navy. He is most famous for demonstrating that lemon juice could be used to prevent scurvy – a huge killer of sailors – in what is sometimes described as the world’s first medical trial.*** He also wrote some important medical texts, including An essay on the most effectual means of preserving the health of seamen, and An essay on diseases incidental to Europeans in hot climates.
One of the ‘diseases incidental to Europeans in hot climates’ he was interested in preventing was guinea worm disease (dracunculiasis/dracontiasis), a disease caused by the worm Dracunculus medinensis (and one of my own specialisms). Acquired by drinking waters contaminated with Cyclops water fleas (copepod crustaceans) which have themselves been infected by the worm, guinea worm is a slow-burn disease, with the worms growing inside the body for a full year before the females make their way to the surface and emerge through the leg or foot in search of water to release their offspring into. It’s now nearly eradicated, but in the 18th century it occurred from the Caribbean to the Aral Sea, but Europeans most often contracted the disease in the Guinea region of West Africa – which is why they called it ‘guinea worm’.
We now know that guinea worm is caused by the combined presence of two separate organisms – the Cyclops and the worm – but Lind, writing a century before the developments of bacteriology and zoology, knew that diseases came from unhealthy environments. Everybody knew that if you went to unhealthy places, the change of environment disrupted your humours and made you sick, and Lind was chiefly interested in how to make travel (and therefore war, conquest and colonisation) safer for Europeans, whom he saw as dangerously out of their element in the tropics. According to Lind, guinea worm:
“has been supposed to proceed from a bad quality in the water of the country, which is in general owing to the woody, marshy soil.”
To medical men of his generation, disease was a problem with the environment – in this case the soil, which contaminated the water of Guinea. And Lind knew that guinea worm came from drinking bad water, because other doctors of his day had observed people who drank such water contracting the disease. Lind therefore set up an experiment:
“In order to know the contents and qualities of these waters, I procured those of Senegal, Gambia and Sierra Leon[ne], which were sent me in bottles, well corked and sealed.”
To Lind, it is self-evident that whatever is wrong with the water will be wrong with any water taken from the place the disease occurs. He sees the environment – the soil – as the driving force behind disease, so it is a perfectly reasonable assumption to him that all water from the dangerous environment of Guinea will contain whatever it is that causes guinea worm. He did not consider the possibility that guinea worm might only occur in certain pools within Guinea. Nevertheless, he writes:
“I could not, however, discover, by the help of a good microscope, the least appearance of any animalcules; nor did any chymical experiment discover uncommon contents or impurities in those waters.”
‘Animalcule’ was a general word for any microscopic organism. Cyclops water fleas, guinea worm’s ‘intermediate host’, were visible under 18th century microscopes – a few of decades later, a doctor called Colin Chisholm would suggest they were the young form of guinea worm.****
A modern scientist might conclude from the absence of any ‘animalcules’ that the bottles never contained any guinea worm-causing ‘qualities’, and go back to Guinea to try again in another pond. Lind, however, came to a very different conclusion:
“All of them, after standing for some time exposed to the open air, became perfectly sweet and good.
Hence I am inclined to think, that the putrefaction of water destroys the live animalcules…which it may contain when fresh; and if such water be permitted to putrify, very wholesome water may be afterwards obtained in Guinea – and thus, supposing the Guinea-worm to be generated from animalcula, or their ova, contained in the waters of the country, their production in the human body may probably be afterwards prevented, by drinking those waters only that have been rendered perfectly sweet by undergoing a previous putrefaction”
Lind thinks that the guinea worm was in the water, but the ‘open air’ had destroyed it. He knows that the disease comes from water, and he therefore believes it impossible that his Guinean water did not contain guinea worm when it left Guinea. His guiding set of assumptions is that disease comes from the environment, and particularly the climate – take water out of the diseased environment, he concludes, and the water will become free of disease.
Which leads him to a very interesting theory:
“The quickest method of freshening such water is, by passing it through a series of vessels, placed under each other, having very small holes bored in their bottoms, so that it may fall in small divided drops, like a gentle shower of rain, through each of them, into a receiver fixed below. The wind, or air, having thus a free passage through the water, divided into small drops, will soon render it wholesome and sweet”
Lind believes that, as the soil and climate of Guinea makes water dangerous, putting this water into a different, more wholesome environment – the open air – will render it safe. This is perfectly reasonable to him, because it accords with everything he knows about disease. If hot climates and ‘marshy soil’ can make water dangerous, it makes sense that clean, cold air, will make it safe.
This may sound far-fetched, but the truth is that Lind’s method would actually protect you against guinea worm – the World Health Organisation recommends that anyone living in areas where guinea worm still exists filter water through a fine cloth mesh before drinking it. Modern scientists and doctors know that this works because it filters out the Cyclops which contain the worm, but Lind lived in a very different world, and drew his conclusions based on what he knew to be true. He wasn’t stupid, or in any way ignorant – he just started with a different set of assumptions from today’s scientists, and arrived at a very sensible conclusion from there.
It’s an underacknowledged truth in science that there are often several plausible explanations for any particular phenomenon. Part of our job as scientists is to work through the possibilities and come up with something which is as close as we can get to what is really going on. But we can’t test everything – we have to assume that we are right about the basics, and that we can rely on our colleagues’ work. We have to build our experiments around what we know to be true about the world – just as Lind did.
I truly believe that our understandings and assumptions in modern biology are basically correct – but it’s worth bearing in mind that back in the eighteenth century, James Lind thought exactly the same thing.
*Which we then try and confirm with statistics, of course.
**I’m aware this is probably an oversimplification, but I’m also not a physicist.
***If you’re interested in learning more about medical trials in the early modern navy, I cannot recommend the work of Professor Erica Charters highly enough.
****Source: my own research, as-yet unpublished. See C. Chisholm, ‘On the Malis Dracunculus’, Edinb Med Surg J 11/42(1815), pp.145-164.
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