CORRESPOXDEXCE. 



625 



plied one upon another. It is to Geoffroy 

 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 : 



"curious effect of gelatine upon 



GLASS. 



" A correspondent sends us the follow- 

 ing 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 Photog- 

 rapher, of November, 1868. 



Singularly enough, just after this date, 

 while experimenting in making my " photo- 

 graphic self-prints from Nature" (an ac- 

 count 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 per- 

 fectly clean. On a close examination, I 

 found that, in removing the ferns and mu- 

 cilage, 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 cor- 

 respondents who may call on me. 



There are numerous very interesting 

 thoughts and queries suggested by the 

 various and yet similar incidents referred 



YOL. y. — 40 



to above. In making sheet window-glass, 

 the workman makes three, and, for very 

 thick glass, four gatherings upon his blow- 

 pipe, creating, as suggested, three or four 

 layers in the finished pane of glass, although 

 not visible to the naked eye. Some work- 

 men reheat the glass after the last gather- 

 ing, 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 disinte- 

 grated by the strong adhesive and contract- 

 ing power of the gelatine and mucilage, over- 

 coming 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 sub- 

 jected to the three successive operations 

 of the grinding, smoothing, and polishing 

 machines. 



The above explanation supposes me- 

 chanical 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 sun- 

 light or the atmosphere. The glasses all 

 contained alkali, in the form of soda or pot- 

 ash, and perhaps some uncombined alkali, 

 which might have formed a chemical com- 

 bination with the acid of the mucilage, and 

 so corroded or disintegrated the surface of 

 the glass. The effect observed was, un- 

 doubtedly, the result of both mechanical and 

 chemical action. 



I found, on inquiry of several dealers 

 in chemicals, that mucilage frequently con- 

 tains 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 constitu- 

 ents of the glass, creating acetate of soda, 

 of potash, or of iron. 



Alum (which is a compound of sul- 

 phate of alumina and sulphate of potash), 

 under the same influence, might be subject 



