From J. S. Burdon Sanderson   12 May [1874]¹

Dear M^r Darwin,

We have made comparative experiments as to the relative digestibility of fibrin & gluten, on the same plan as those we made before, of which I have written the results on the next page. They shew that gluten is much more difficult to digest than fibrin.²

I propose tomorrow to try after subjecting gluten to digestion, there is any remainder i.e—whether gluten contains any indigestible constituent, for it must I think be regarded as a mixed substance

D^r Klein³ will tell you what he has observed in the bones we have digested

Very truly yours | J B Sanderson

Two comparative experiments were made with the same digestive liquid, one with fibrin the other with gluten, each lasting four hours as in the previous series.

The digestive liquid contained 8^.8 of pepsin-extract & 6^.4 of hydrochloric acid in 1000 C.C. of water. The quantities digested in each case were

Hence the quantity of gluten digested was to that of fibrin as 40^.8 to 100.⁴