I can’t believe it’s not…

Knife in butter dish

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The analytical techniques revealing the true identity of what you spread on your bread

A letter to Nature by C Simmonds described this illegal activity as having ‘developed into quite an industry, having its own factories and its own chemists, and conducting its operations on a scale which, for a furtive dishonest business, is really of remarkable magnitude’, whose ‘traffic has flourished in spite of all attempts at suppression’ likely due to ‘considerable profits’.