This paper aims at exploring the history of a scientific fact, most widely known as the mechanical equivalent of heat. When did this "truth attested by authentic testimony" came to be known as a fact, credited as scientific? The fact to be investigated here was and is an everyday experience known since the existence of human kind: when two surfaces are rubbed against one another heat is produced. However only in the late 18th century did the relation between heat and mechanical force become a matter of intensive natural philosophical investigation, and not before the early 19th century was it expressed as a numerical fact. It soon became the building block of the new British science of energy and was even regarded as proof of the principle of conservation of energy. It then achieved the status of constant of nature and finally around 1900 was named the golden number of the century. The paper therefore covers the period from the late 18th until the end of the 19th century, a time of considerable change within the development of the physical sciences. Since the fact in question is one which can be expressed by number only, the narrative is about the establishment of precision measurement in the 19th century "exact sciences", and especially about quantitative experimentation as a specific form of experience.