In a paper published in the Notes & Records of the Royal Society in November 2010 (Alfonso-Goldfarb, Ferraz and Rattansi, Lost Royal Society documents on 'alkahest' (universal solvent) rediscovered, vol. 64: 435-456) we discussed the rediscovery of documents connected with three meetings of the early Royal Society (1661) devoted to an experiment to demonstrate that a counterpart of the alkahest or universal solvent was present in the newly discovered lymphatic vessels. Although the minutes published in Birch reproduce the Minutes, it was difficult to reconstruct the discussions without access to the papers mentioned in these Minutes. However, those primary sources had themselves been filed and refilled by Royal Society cataloguers over the subsequent centuries, as the sciences became increasingly specialised. The task of recovering them and of identifying the author proved laborious and often led into blind alleys.

Specialisation among historians of science, too, contributes to difficulties in researching a subject like the ‘alkahest’. The word originated with Paracelsus, as a remedy for liver disease, but it was transformed into a ‘universal solvent’ by his great ‘iatrochemical’ successor, J.B. van Helmont. For him it was both a proof that all earthly things are made from water and as providing the most effective remedy for all diseases (since it recovered from all substances their therapeutically most effective constituents). R. Boyle provisionally accepted the belief and H. Boerhaave was to treat it at length in his Elementa chemiae.

Placing the Royal Society discussions in their contemporary scientific and cultural context demanded immersion in the development of post-Vesalian anatomy and physiology which led to the discovery first of Harveian blood circulation and then of the lymphatic system. T.L. Kohlhans, whom we identified as the author of the ‘animal alkahest’ paper, could then be seen as arguing that the purpose of that system was not merely to carry pure water or ‘lymph’, but to extract the ‘seminials’ from the ingested nutriment (chyle) and distributing them to bodily parts. Kohlhans was thus using the Helmontian alkahest to answer a puzzle about the function of the lymphatic system.

Our researches for the recovery of the lost papers illustrate vividly the difficulties that changing subject classifications raise for research in primary sources and, in conclusion, we shall raise some wider considerations based on our own experience.