Some people become obsessed with tracking their weight, carefully counting every calorie they eat and burn through exercise. They even weigh themselves before and after every meal. In 17th century Italy, there was a man similarly fixated on measurement, though not out of vanity or a desire for the perfect physique. His motivation was purely scientific.
The human body loses significant amounts of heat and moisture through the pores in the skin. Water is also lost through breathing, as moisture-laden air is exhaled. This type of water loss is known as "insensible perspiration," which differs from regular sweating. While sweat contains solutes, insensible perspiration is the evaporation of pure water. It’s also invisible and undetectable, hence the term "insensible."
Photo credit: Amit Chowdhury
Modern studies show that an adult human loses approximately 400 ml of water per day through evaporation from the skin, with a similar amount lost through breathing. While water loss through respiration varies and can be influenced by the humidity of inhaled air, the water lost through insensible perspiration from the skin is inevitable and cannot be reduced.
The earliest known scientific study of insensible perspiration was conducted in the early 17th century by Italian physician Santorio Santori, better known as Sanctorius of Padua. Often regarded as the father of experimental physiology, Santorio spent over 30 years meticulously tracking his own bodily processes, measuring how much he ate and excreted daily. He observed that a significant portion of the weight from his food intake was not excreted in his feces or urine, nor was it reflected in his body weight. Santorio theorized that this mass was lost through the pores of the skin as moisture. He called this phenomenon perspiratio insensibilis or insensible perspiration.
Santorio weighed himself every day before and after various activities, such as eating, defecating, urinating, and even sexual activity. He also carefully measured the weight of his food, feces, and urine. To aid in his experiments, he designed a special weighing chair, allowing him to record changes in his body weight with great precision. He described and illustrated this invention in his work Canon of Avicenna (Commentaria in primam fen primi libri Canonis Avicennae), published in 1625:
The chair is arranged as it appears in the figure (see Fig. 1), in which the steelyard is suspended from the beams above the dining room, in a hidden place because of the nobles, as it renders the room less appealing, and because of the ignoramuses, to whom all unusual things appear ridiculous. The chair remains lifted from the floor at a finger’s height, stable in such a way that it cannot be easily moved; when, due to the ingested food, one reaches the expected weight and the measure previously set, then the outermost part of the balance ascends a little and contemporaneously the chair descends a little. This descent immediately indicates to the sitter that he has arrived at the stabilized quantity of food; which quantity or weight of salutary food is advisable for somebody, and how high the insensible transpiration in the individual bodies should be, one weighs comfortably with the chair.
Santorio Santori
Santorio’s weighing chair hung from a beam attached to a large steelyard. Before eating, he would set the balance to correspond with the amount of food he intended to consume. Then, seated in the chair, he would eat until the chair’s balance dropped to the predetermined measure, at which point he would stop eating.
The primary goal of the weighing chair was to determine the appropriate amount of food each person should eat to maintain health. Santorio believed that carefully monitoring bodily discharges was crucial for preserving well-being. He theorized that health was achieved through an ideal balance between ingestion and excretion, meaning the amount of food taken in should correspond to the amount of liquid and waste expelled from the body. His experiments with the weighing chair revealed that a significant portion of bodily excretion occurs invisibly, through the skin and lungs—an observation that became central to his 1614 publication, De Statica Medicina.
Santorio didn’t limit his experiments to himself; he also measured other individuals using his weighing chair. He weighed them before and after meals, as well as at various intervals throughout the day and night. By meticulously recording the exact amount of urine and feces expelled, and combining this data with the known quantity of food consumed, Santorio was able to estimate the amount of perspiratio insensibilis that was released overnight. As his experiments progressed, he began to factor in additional variables such as climate, sleep, exercise, age, and even emotional states, carefully adjusting these conditions to refine his understanding of the ideal balance between ingestion and excretion.
Santorio accurately concluded that perspiratio insensibilis occurred through the pores of the skin, which he described as being "like a net" that covered the entire body, allowing moisture to escape. He also noted that some of this invisible moisture was expelled through respiration, a fact easily demonstrated by "breathing upon a glass" and observing the condensation.
Santorio’s experiments were not merely designed to measure perspiratio insensibilis; he also envisioned them as a diagnostic tool, capable of predicting the onset of diseases based on fluctuations in body weight. His hypothesis was that under normal conditions, the body maintains a relatively stable weight. Therefore, the consistent dispersion of matter through insensible perspiration indicated a healthy state, while sudden changes in this balance could signal the emergence of a hidden illness. Santorio proposed that by calculating perspiration, physicians could determine the appropriate dosage of drugs for each stage of a disease and gauge its severity.
While Santorio overestimated the practical applications of his findings, the significance of De Statica Medicina in the history of medicine and science is undeniable. His weighing experiments introduced the concept of quantitative research into physiology, marking a pivotal shift from theoretical to experimental approaches. Santorio’s work demonstrated that the body’s most essential processes, by which it maintains itself, are quantifiable and must be studied through precise measurement and observation rather than abstract reasoning.
Santorio went on to create more than thirty inventions, most of which were in the field of medical science. Among his notable contributions were an early thermometer for measuring body temperature and the pulsilogium, a device designed to measure pulse rate. The pulsilogium is considered one of the first precision instruments in medical history, revolutionizing the way doctors monitored a patient’s vital signs.
Many of the concepts, tools, experiments, and methodologies that are now viewed as foundational to early modern science were significantly influenced by Santorio’s work. His inventions laid the groundwork for more accurate diagnostic methods, and refined versions of these instruments continue to be used in clinics around the world today.
References:
# Teresa Hollerbach, “The Weighing Chair of Sanctorius Sanctorius: A Replica”
# Fabrizio Bigotti & Jonathan Barry, “Santorio Santori and the Emergence of Quantified Medicine, 1614-1790”
# “Santorio Santorio (1561-1636)”, Vaulted Treasures
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