The ‘Heliotype’ was a collotype process developed, patented, and introduced by Ernest Edwards in 1869. Edwards’s technique involved adding chrome alum to the gelatin as a hardening agent. The process was used for book illustrations this being a fine example.

Stillman, W. J., editor Poetic Localities of Cambridge James R. Osgood and Company, Boston, 1876. 8vo, 41 pages.
12 Heliotype illustrations from photographs of scenery around Cambridge, Mass. (Some copies have different masking of one or two images, perhaps indicating a second, undated printing.) The noted painter and photographer William Stillman’s only book illustrated with collotypes.