If on a cold, polished metal, as a new razor, any object, such as a wafer, be laid, and the metal be then breathed upon, and, when the moisture has had time to disapper, the wafer be thrown off, though now the most critical inspection of the polished surface can discover no trace of any form, if we breathe once more upon it, a spectral image of the wafer comes plainly into view ; and this may be done again and again. Nay, more, if the polished metal be carefully put aside where nothing can deteriorate its surface, and be so kept for many months, on breathing again upon it the shadowy form emerges.

This passage is from Chapter V., "Conflict respecting the nature of the soul. -- Doctrine of emanation and absorption", of History of the Conflict Between Religion and Science by John William Draper. The original is from 1874, but I am quoting page 132 of the "Author's Edition, limited to one thousand copies", Appleton's Scientific Library. Comparing the binding with other titles from D. Appleton & Co., I estimate it dates from the 1890s.

Draper is giving analogies for "ganglionic impressions" (learning and memory). Some other analogies he gives are latent images on photographic plates and afterimages.

Being a crusty materialist, I conjecture the following mechanism: water molecules from the breath are adsorbed on the metal. The visible layer soon evaporates, but a thin, possibly monomolecular layer remains. When one breathes on the blade again, this adsorbed layer serves as a substrate for subsequent condensation. This conjecture could be disconfirmed by subjecting the blade to some kind of outgassing treatment (say, exposing to a vacuum, or heating, or both), which I predict would make the 'image' disappear.

However, apostate Romantic that I am, I cannot let the image go: the breath upon the blade. The breath (Hebrew ruach, Greek pneuma, Latin spiritus) upon the blade, an instrument well suited to making the breath expire. I am imagining a Samurai sword or a blade of fine Damascene steel, not a mundane Sweeney Todd sort of straight razor.