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to the chemical interchange called isomorphism, giving up its potash for some of the soda in the glass. These new compounds, if soluble in water, would be removed in washing off the mucilage, or, if insoluble, would be torn away when pulling off the paper attached by the adhesive gum.

In view of these facts, it would be well not to run the risk of possible injury to any of our valuable windows by affixing notices upon them with mucilage.

My correspondent thinks he noticed a continued or increasing corrosion, or disintegration, of the surface of the plate-glass after the removal of the mucilage. On examination of one of the pieces sent to me, I found a discoloration, occasioned by what we call "rust," in a part not affected by the mucilage, and this was probably what our friend observed.

There are glasses sometimes made with an excess of uncombined alkali which effloresces upon the surface, sometimes before it is placed in the window, and sometimes at a later period. This, of course, causes a series of infinitesimal holes or furrows in the glass, whose surface by the reflection and refraction of the rays of light presents, like the mother-of-pearl, all the colors of the rainbow when held in a certain position, and at the right angle for such effect. This is one kind of what is called stained or rusty glass. In other specimens the disintegration goes beyond the infinitesimal character and is plainly visible, making the glass appear as if fire-cracked, and in its ultimate effect producing the appearance, on one or both surfaces, of ground glass.

In one of these stages of rust or disintegration I find a piece of the glass referred to by my correspondent.

In reference to the matter of rust on glass, for the comfort of my readers and of my New York friends who are dealers in window-glass (which was my business for many years), let me say that this difficulty or defect is not an every-day trouble, but only one of the incidents or curiosities of glass-making. A good glass-maker knows how to avoid it, and a good glass-dealer can, in many instances, remove the first appearance of rust or stain on plate or sheet glass by a skillful and nimble use of dilute hydrofluoric acid and rouge or oxide of iron.

Glass-making and glass-dealing have their troubles, and this of occasional rust on glass is one. But all other kinds of business have their trials, and they all have their points of interest and satisfaction. In the study necessary to remove troubles and avoid mistakes, comes half the pleasure of life. If we had no obstacle in the path of our business, we ourselves would rust out.

In closing my article, let me add that I give my opinions only as humble suggestions, after such examination as I have been able to give to the subjects referred to.

In the light of new experiments, and the progress of scientific research by those who make it the business of life, the theories of to-day may be set aside by the revelations of to-morrow. If "an undevout astronomer is mad," so must be an undevout chemist, or student in any domain of natural science, who will not humbly bow before the wise and wonderful workings of the Great Maker of the universe. We cannot fully understand or explain them now. We can only look "through a glass darkly." We can see and enjoy the light of the sun, but, though great and leaned volumes be written, who can fully explain all the laws and all the wonders of light, one of the gentlest and most ethereal, and yet one of the most interesting and powerful elements of the universe? We must be content to labor and to wait.

Boston, July 20, 1874.




THE comet has come and gone, and again, raised the perplexing question as to what such bodies are made of, and what are the most subtile forms of matter diffused through the celestial spaces. Of the great moving masses which compose our own system, from the sun—1,000,000 times larger than the earth—to the little asteroids—250 miles in diameter—and from these down to the meteorites