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The Chemistry Of Epsom Salt

When the virtues of Epsom Waters were first recorded chemical small analysis was in its infancy; magnesium as an element was not discovered until nearly a century later. Methods of measurement were not standardised, and there are some uncertainties in the conversion of the units quoted into their modern equivalents.

So Aubrey, the first to carry out some experiments with the water around 1654 (not published until 1719) reported that he evaporated about a gallon, which yielded "a sediment of flakey stuff, of the colour of Bay-Salt in loose flakes, as much as fill'd a Tobacco-Box". He gave it to old Dr. W. Harvey, "who thought it to be a sort of gray Nitre; but he spoke at random."

Later reporters became more accurate in their statements, and the table below is an endeavour to convert the chemical contents reported into modern units: grammes per litre.

We should realise (as contemporary observers noted) that the salt contents would vary not only with the amount of rainfall, but also with the depth of water at which the sample was taken.

Because Epsom water was no longer of importance in the 19th century, there are no comparisons of its magnesium sulphate contents with that of other spa waters, when these were meticulously analysed a century or so ago.

In spring 1989 the Borough of Epsom and Ewell arranged for a professional analysis of a sample of water taken from the old well. The result is included in the table below. It appears that the Epsom Salt contents are now much smaller than it apparently was in the 18th century.

Total Solids MgS04


1695   Grew Epsom Old Well                       6-10 drachms/gallon           6.2-10.3

1699   Allen Epsom Old Well                       7 drachms/gallon                 7.2

1711   Allen Epsom New Well Vi                 drachm/pint 4.1

1725 Hoffmann Epsom Vi quentgen/medic. pound 3.9

1751 Hales    Epsom 34 grains/pound (Avoir.) 4.8

Lambeth' 40 Vi grains/pound 5.8

Acton 44 grains/pound 6.3

Cobham2 60-68 grains/pound 8.6-9.7

Jessop's Well 3 82 grains/pound 11.7

Cheltenham 42-74 grains/pound 6.0-10.6

1756 Lucas Epsom Old Well 40 grains/pint 5.5

1805 Saunders Epsom quoting Lucas 5.5

1987 Nescot Epsom Old Well of which 4/6-5/6 MgS04 3.2

1989 Moir & Palgrave Epsom Old Well _

1831 Granville Sea Water 2.295 grains/1000 grains