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Rh one upon another. It is to Geoffrey that we owe this observation."

This was the only item of the kind which I ever found in the old books, and I had my doubts of its accuracy, until I read, in the London Photographic News of July 17, 1868, the following article:

"A correspondent sends us the following account of a curious result:

"Having, for experimental purposes, poured a thick solution of gelatine upon a number of glass plates, three of them were set aside upon a shelf for some months; and one day, upon looking at them, I found that, in all three cases, the gelatine had separated from the glass, bringing away the whole surface of the glass plates in shivers, which firmly adhered to the gelatine. The surface of the glass was left full of ruts, like water-worn stones. I suppose it to be caused by the strong contraction of the gelatine, and its firm hold upon the glass."

I wrote a short notice of these two similar facts for the Philadelphia Photographer, of November, 1868.

Singularly enough, just after this date, while experimenting in making my "photographic self-prints from Nature" (an account of which I have sent you in my little pamphlet), I noticed a similar phenomenon.

You will recollect that I place leaves and ferns upon glass with mucilage, and print their forms upon sensitive paper by exposure to the sunlight. After the ferns are dried up, I clean the glass for further use. In washing one of these glasses, it was impossible to make the surface perfectly clean. On a close examination, I found that, in removing the ferns and mucilage, the latter had taken off a portion of the glass, so that I could distinctly observe, on the crowded surface, the outlines of an anchor (which was the figure produced), and the forms of some of the individual ferns. I have this curious specimen—not of plate, but of sheet glass—in my cabinet, and will show it to you or any of your correspondents who may call on me.

There are numerous very interesting thoughts and queries suggested by the various and yet similar incidents referred to above. In making sheet window-glass, the workman makes three, and, for very thick glass, four gatherings upon his blowpipe, creating, as suggested, three or four layers in the finished pane of glass, although not visible to the naked eye. Some workmen reheat the glass after the last gathering, in order, by what is called "burning over," to make the heated ball more uniform and homogeneous. The glass is then more easily and perfectly annealed, and more easily and safely cut by the diamond. The sheet-glass, named in the curious incidents related above, was probably of a kind not "burnt over" and perfectly homogeneous, and, for this reason, more easily disintegrated by the strong adhesive and contracting power of the gelatine and mucilage, overcoming the cohesion of the atoms and layers of the glass.

While crown and sheet glass have an original fine surface, that of plate-glass is softer and more easily affected, because it is an artificial one, which has been subjected to the three successive operations of the grinding, smoothing, and polishing machines.

The above explanation supposes mechanical action only. But, it is possible, a chemical action took place also, especially in the plate-glass, in the formation of some acid, by the fermentation of the gelatine or mucilage, when under the influence of sunlight or the atmosphere. The glasses all contained alkali, in the form of soda or potash, and perhaps some uncombined alkali, which might have formed a chemical combination with the acid of the mucilage, and so corroded or disintegrated the surface of the glass. The effect observed was, undoubtedly, the result of both mechanical and chemical action.

I found, on inquiry of several dealers in chemicals, that mucilage frequently contains acetic acid or alum, to prevent the formation of mould. In such cases, the acetic acid might easily form a chemical attachment, under the warming influence of the sun's rays, for some of the constituents of the glass, creating acetate of soda, of potash, or of iron.

Alum (which is a compound of sulphate of alumina and sulphate of potash), under the same influence, might be subject.