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  • Armel Cornu

How do we know when water is safe to drink? The early modern origins of water analysis

Updated: Sep 22

In areas where clean drinking water is available on tap, it can be easy to forget what a monumental achievement water safety is. This luxury — still far from being available to all today — rests on the knowledge of what makes water safe to drink. In this piece, I want to bring attention to an area of science which became crucial in the advent of water safety, before plumbing infrastructure reached most homes. I will describe how chemistry, in its early days, developed a method which could evaluate the contents of waters.

Pre-industrial populations consumed large quantities of water. Exact amounts are difficult to estimate, but people did drink plain water, and used it to cook daily. Though water safety was never fully guaranteed, many had an intuitive sense of what constituted clean water. Thousands of years of medicine and collective experience had helped categorise waters on a spectrum from safe to dangerous. For much of the mediaeval and early modern period, the classification read as such: stagnant waters from marshes and hot climates were viewed as the least healthy, likely to bring about fevers and even epidemics. The waters of rivers and wells were viewed as healthier, though they were known to have some risks. Particularly, they could be subject to external contamination. Finally, the flowing water of springs was seen as the highest possible water quality.

This classification was functional but imprecise. If a disease suddenly began to spread in a village, how could town officials reliably know whether the local well was to blame or if the illness came from elsewhere? By the early modern period, in the face of growing urban populations and concerns over the spread of epidemics, demand grew for a secure method of assessing water quality. Chemists, who were at the time part of a relatively new field, were eager to try their hand at the task. If they were able to create a method for water analysis, they could be more trusted by the general public, and could further establish the usefulness of their discipline.

It was well understood that water itself never varied in quality. Water was — according to early modern theory — an element, and an element in its pure form was indistinguishable regardless of where it was found. What made waters healthy or disease-ridden were the components dissolved in those waters. Though there was no understanding of germs or bacteria, disease was known to proliferate in warm and unmoving waters. Conversely, some waters could contain healthy substances, like minerals or salts. Chemists therefore needed a method which could identify the various substances present in waters, down to the smallest quantities.

Several analytical techniques were rediscovered, improved, and innovated throughout the early modern period. It is estimated that several hundreds of practitioners participated in this work and they often collaborated with one another. These practitioners wrote books about their works which could be shared and discussed among the wider community of water analysts. Because the analysis of waters had important stakes regarding health, most of these practitioners were medically trained, either as physicians or as apothecaries.

By the late eighteenth century, the chemical analysis of water had become a largely standardised practice, composed of several steps. Ideally, the analysis had to begin on site, where the water was located. There, the analyst could begin the work, not with instruments, but with their senses. Smelling, tasting, and feeling the water with the hands could provide valuable information about the water’s composition. Other everyday procedures could also be performed, like trying to wash a piece of cloth or cooking vegetables in the water. Following these tests, the analyst could begin using instrumentation. An important metric was to measure the weight of the water using an areometer. This instrument indicated how light or heavy the water was, when compared to pure distilled water.


Lavoisier’s collection of areometers, visible at the CNAM museum (photo by author).



The next step came about only in the middle of the eighteenth century, as chemists gained a heightened interest in gases (called ‘airs’ at the time). They attempted to isolate the air that was often dissolved in waters using dedicated instrumentation. For example, an analyst could fit a bladder atop a bottle containing some water, shake it, and then collect the gas trapped in the bladder. Further tests could then reveal the nature of this gas. This step was particularly important when analysing naturally fizzy springs, but it was less needed when analysing the water of wells or rivers.

Following these steps came the use of reagents. This was a widely varied and controversial step, but it was nevertheless viewed as important. Reagents came in all forms. The most common was the oak gall tincture, a red liquid extracted from oak trees which turned black in the presence of iron. Another common reagent was the violet syrup, which acted as an acid-base indicator. But these two substances only represent a sample of the hundreds of different reagents used by early modern chemists. They used salts, acids, alkalis, as well as a host of different plant powders, metal dissolutions, and sometimes curious products (to our modern sensibilities) such as blood or crayfish eyes. Analysts poured the water they were analysing into a number of cups or glasses, added a reagent in each, before watching for any transformation to occur. On average, chemists reported using around eight different reagents per analysis.

Once an analyst had used the reagents, they had a rough idea of what kinds of substances were present in the water, but they had no idea of the quantities of each of these substances. This is when the final and most intricate part of the analysis came into play: the evaporation. This was carried out in a variety of ways. Distillations were sometimes performed, but the most common technique was a dry evaporation. The analyst gathered large quantities of water (between a few litres and a few dozens of litres typically) and slowly evaporated it. Some filtrated the water many times throughout the process; others preferred to wait until all water had evaporated to treat the residue in one go.


The treatment of that residue was the most delicate part of any water analysis. It typically involved a series of solvent extractions. The residue was rinsed with vinegar, hot and cold water, and other such liquids in order to separate the different components of the solid residue. Once shaken with the residue, the solvent could be evaporated and filtered again. The chemist then laboriously attempted to get each part of the separated residue to cristallise. They used further analytical techniques in order to identify each solid substance, by tasting the residue, attempting to calcinate it, observing it with a magnifying glass, or checking for movement when approaching a magnet. Once all of the different parts were separated and identified, the analyst could weigh them, and thus calculate a concentration for each substance.


Model for the cristallisation of different salts. Encyclopédie, Volume III, Planche XVII. Via Édition Numérique Collaborative et CRitique de l’Encyclopédie (ENCCRE), 2017.


Virtually all chemists agreed that the chemical analysis of waters was a difficult endeavour. The influential Swedish chemist Torbern Bergman titled the section of his book on the subject: “An accurate Analysis of Waters is justly considered as one of the most difficult Problems in Chemistry” (Bergman, 1784). Consequently, chemists tended to multiply the steps, so as to obtain an accurate result by the convergence of several different experiments. They often spelled out the limitations of their analytical results, and kept some transparency regarding the observations that they could not interpret.

This method was initially applied to mineral water springs. Since mineral waters contain relatively high quantities of substances, they were marginally easier to decompose than other kinds of water. But as the method became more and more precise and analysts collaborated with each other in order to improve their technique, the analysis of river and well water became more common. Chemical analysis thus slowly became a trusted method that many believed could vouch for the safety of any source of water, and could warn the public against consuming contaminated sources of water. Chemists, in that sense, performed an important public health mission by appraising the quality of waters, the legacy of which can still be seen today.


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Author bio: Armel Cornu recently graduated with a doctorate in History of Science from the University of Uppsala, Sweden. As of September 2022, she will be a postdoctoral fellow at the Science History Institute in Philadelphia, Pennsylvania.


This article is based on the research conducted in her Ph.D thesis: Cornu, Armel, Enlightening Water. Science, market & regulation of mineral waters in eighteenth-century France, Uppsala Studies in History of Ideas 55, Uppsala: Acta Universitatis Upsaliensis, (2022), available Open Access here.


See chapter 6 for further descriptions on the medical background of analysts, and chapters 7–8 for full details on the chemical analysis of waters. Additionally, see appendix 4 and 8 for a full list of the reagents and instruments used in water analysis throughout the eighteenth century.

References and further reading

On water and its place within health systems in history:

-Jouanna, Jacques. "Water, Health and Disease in the Hippocratic Treatise Airs, Waters, Places". In Greek Medicine from Hippocrates to Galen, (Leiden, The Netherlands: Brill, 2012).

-Marie-Thérèse Lorcin, “Humeurs, Bains et Tisanes : L'eau dans la médecine médiévale, L’eau au Moyen Âge,” Senefiance 15 (1985).

-Roy Porter, “The Medical History of Waters and Spas,” Medical History 34, no. 10 (1990).

-Brennan, Thomas Edward. Public Drinking and Popular Culture in Eighteenth-Century Paris. Princeton Legacy Library, 1988.

-Chaplin, Joyce E. “Why Drink Water? Diet, Materialisms, and British Imperialism.” Osiris 35 (2020): 99–122.


On the chemical analysis of water:

-Noel G. Coley, “Physicians and the chemical analysis of mineral waters in eighteenth-century England,” Medical history 26 (1982).

-Michael Bycroft, “Iatrochemistry and the Evaluation of Mineral Waters in France, 1600– 1750,” Bulletin of the History of Medicine 91, no. 2 (2017).

-Euzen, Agathe, and J-P Haghe. “Drinking Water from the Seine in the 18th Century, or the Emergence of the Filtration Fountain.” 5th IWHA conference, Pasts and Futures of Water, Tampere, Finland, 2006.

-Hamlin, Christopher. A Science of Impurity, Water Analysis in Nineteenth Century Britain. Berkeley, Los Angeles, Oxford: University of California Press, 1990.



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