Editorial note — Chapter XX · THE SAMPLING AND ANALYSIS OF TANNING MATERIALS.

[196] Procter and Blockey quoted experiments at the Leeds Conference, proving that gallic acid and some other non-tanning substances were largely absorbed by the hide-powder filter, though probably not permanently retained by leather; while the error, though still considerable, was much less when the chromed hide-power shake method was employed. Where only gallotannic and gallic acids are present, as in the case of sumach and commercial gallotannic acids, the most accurate quantitative estimation is probably that by the Löwenthal method carried out as described L.I.L.B., p. 123, but considerable skill is required in its execution.