stituents of each specimen, a good foundation would be laid for a thorough scientific investigation of the subject. This has never yet been done, and in the absence of such knowledge, we can only theorize upon the results which we witness.

While I may not theorize, I may help others to do so by stating the interesting part which some metallic oxides play in coloring, and the oxide of manganese in decolorizing glass. In almost all kinds of window glass and glass-ware, materials are necessarily used which are not perfectly and chemically pure.

The sand, the carbonate or sulphate of soda, the lime and other constituents, one or all, contain slight impurities, and almost always oxide of iron.

The protoxide of this metal gives glass a bluish or bluish-green tint, the peroxide a yellowish, and a mixture of the two, in which condition it is generally found in glass, produces the almost universal greenish tints which we witness in the glass of commerce. To correct in some measure the coloration by iron, a small proportion of oxide of manganese, called "glassmakers' soap" is put into the "batch," or glass mixture. The natural tint produced by manganese in a high state of oxidation is purple or violet, and we cannot produce these colors without its use. In proportion as it is deprived of oxygen, it loses its coloring power, and when it reaches the state of protoxide, it becomes nearly colorless. *

Now in the reaction which takes place in the melting-pot, the manganese gives up a portion of its oxygen and its coloring power to the iron, which is converted into peroxide, which gives a yellowish color to the glass.

This yellow color is complementary to whatever of purple coloring power is left in the manganese, and is therefore neutralized, and the glass comes out of a light color. When the sunlight strikes this glass, the nice balance between the oxygen of the iron and the manganese is disturbed, and there ensues, even in so solid a body as glass, an interesting contest for the possession of the coveted oxygen, and the conquering element is known and announces its victory by a display of its special colors. Every change of color or tint involves the wonderful condition, invisible to human eyes, of a molecular or inter-molecular movement of the atoms of this solid body, and it is probably attended by two circumstances: an interchange or redistribution of the oxygen among the constituents of the glass, and the development, in consequence of this

redistribution, of the special coloring power of the metallic oxide which has the greatest affinity for oxygen, or stands in the greatest proportion to the other constituents.

And so in some specimens the yellow of the iron predominates, in some the purple of the manganese, and in others, the yellow at first appears, and afterwards a yellowish purple and purple fully developed.

While these suggestions may help to explain the sunlight coloration of the so-called colorless glasses, they may apply also to colored glass, inasmuch as all of the sensitive specimens contain a certain proportion of iron, or manganese, or both.

If this coloring power of the sunbeams has not been generally known, it is because it cannot readily be seen by ordinary observers, except in such as the purple-tinted windows of Beacon street, and even then it may sometimes require the background of a light colored curtain.

The discovery of the defect is an annoyance to both consumers and glassmakers. When our Beacon street friends could no longer obtain similar glass to replace broken panes, the insertion of colorless ones gave their windows the interesting checker-board appearance which we witness to-day. When the glassmakers found the defect a matter of scientific observation and experiment, and their results published at home and abroad, they remedied the difficulty in a measure by using purer materials, or reducing the proportion of manganese in their "batch," or giving up its use entirely, preferring to have the glass assume its natural and more permanent color, even if it be a little greenish or bluish, rather than by "doctoring" the mixture, to adopt the glass-makers' term for the use of manganese,― to have it light colored to-day and easily affected by the sunlight of to-morrow.

This improvement is of especial importance to photographers, who, in all operations requiring short exposures and all the light possible to obtain, would avoid the use of any glass in their skylights which, after a few months or years of exposure, will be robbed of a great proportion of its power to transmit the chemical influence of sunlight by a change to a yellow or purple tint, which in time might cut off almost as much actinic effect as if it had been ground or enamelled on one of its surfaces.

I have made some photographic experiments to show this deteriorating effect, by exposing sensitive paper under glasses of the

original colors, and those of the same kind, changed by sunlight exposure, and witnessing the perceptibly different shades of darkening produced. This action of sunlight must not be confounded with that called "rust," or "stain," which is occasioned in some glasses having an excess of alkali in their composition, by exposure to the atmosphere, and manifests itself in two ways; first, by a disintegration and roughening of the surface, sometimes producing all the effects of ground glass; and secondly, by an efflorescence and apparent formation of an infinitesimal coating of oxide upon the surface, on which the play of the sun's rays produces all the colors of the rainbow, as with the action of light on the infinitesimal grooves of mother-of-pearl. This is simply surface action, whereas the action of sunlight permeates the whole body of the glass wherever the rays directly strike it.

I might refer to many other points in connection with my subject, but my time will not permit and I hasten to exhibit the results of my experiments, asking my hearers, who are so much better versed in scientific studies than myself, to give me their light in ascertaining the causes and exact operations of this interesting power of the sun's rays to paint the products of art, as they do so beautifully and wonderfully the works of nature on the mountain, in the forest and field.

The large number of specimens of colored and colorless glasses here exhibited show the coloring effect of exposure to sunlight. A tangible illustration of this subject is found in the purpletinted windows of some of the Beacon- street houses, of which the specimens exhibited show the original color and the changes produced by sunlight exposures varying from one day to fifty years. These examples also show how this action can be turned to interesting account in impressing upon colored and colorless glasses the forms of leaves and ferns, and in printing inscriptions and mottoes. It is a species of photographic work with sensitive glass, instead of sensitive paper, the sun showing itself a most excellent printer and developer, and, indeed, the only true photographer in colors.

THE DISCOVERY OF OXIDE OF ANTIMONY, IN EXTENSIVE LODEs, in SONORA, MEXICO. By E. T. Cox, of Oakland, Cal.

Up to the present time, the antimony of commerce has been mostly obtained by the reduction of the sulphuret; and though this ore is widely distributed over the globe, it is, as a rule, associated with a variety of mineral substances that obstruct reduction and add to the cost of purifying the metal. These sulphurets are also found in such sparse quantities, that the metal usually commands from three to four times the price of lead and fully as much as that of tin or copper. At present, the principal supply of sulphuret of antimony, for the English smelters, is obtained from Algeria, Spain and Ceylon. Small quantities of oxidized antimony ores have been found in portions of Europe, and in Ceylon, but at no time in such quantities as to elicit special attention from the smelters. When, therefore, about a year ago, I called the attention of English metallurgists and smelters to the occurrence of vast lodes of almost pure oxide of antimony, in the district of Alta Sonora, Mexico, thirty miles from the Gulf of California, it seemed too marvellous for their belief. A company of gentlemen of Boston, Mass., now have control of these antimony mines, and the ore will soon be in the hands of the smelters.

The geological features of the country, where this ore abounds, is similar to that of southern Arizona. The mountains are in short, narrow ranges, having, for the most part, a northerly and southerly trend. Their crests are either rugged or well-rounded cones, according to the nature of the rocks forming their mass. Between these ranges, we have what is called mesa or table-land; the latter is formed of the débris of the mountains. This material is of so loose and porous a nature, that the small amount of rain which falls sinks through it and leaves the land dry and arid. As far as I have been able to make out the order of the rocks forming these mountain chains, we have first granite, and this is flanked by sub-carboniferous limestone, in most places so crystalline as to obliterate all traces of fossils. Protruding through these and forming the mountain peaks, we have porphyry, quartzites, basalts, diorites and trachites.

The country in the immediate vicinity of the antimony mines is quartzite and limestone, The ore lodes are from four to twenty feet wide, and exploration work, carried to a depth of thirty feet,

shows that the fissures are filled, from wall to wall, with the oxide of antimony, almost pure and remarkably uniform in character. The course of the lodes is nearly north and south; the pitch is 90° to the east. The area of country, over which the ore is found, may be roughly stated to be five or six miles long and half a mile or more wide.

The Boston company controls nine mines, each of which is a full Mexican claim, 800 metres (2624' 8") long and 200 metres (656'2") wide. On three of the mines, the crop, which is solid oxide of antimony, like the samples exhibited upon the table, stands up boldly above the general surface and may be traced along the claims for many hundred feet. As stated above, the ore, so far as explorations have exposed it, is almost pure oxide of antimony, the little impurity it contains being silica. The fire assay shows it to contain from 60 to 70 per cent. of pure metal, and I have estimated the entire lode to average 50 per cent. By selection the average may be augmented.

On going down to a greater depth on the lode, it is possible that the oxide may give place to sulphides, but thus far there is not the slightest evidence of any change. This discovery is destined to produce a marked influence upon the production of metallic antimony and greatly to extend its uses.

Note.-Prof. S. P. Sharples, of Boston, said that he had examined many specimens of the oxide of antimony received from Prof. Cox. These varied in color from almost white to a very dark brown. The specific gravity of one of the purest specimens is 5.07 and it contained 5 per cent. of water and 75 per cent. of antimony. This composition and specific gravity approach very closely that of stibiconite.

The mineral is only very slightly soluble in hydrochloric or nitric acids or aqua regia. Fusion with bi-sulphate of soda only partially resolves it. It is, however, readily and easily decomposed by fusion in a platinum crucible with carbonate of soda.

This oxide of antimony has, hitherto, been found only as slight coatings in other antimony minerals and it has been very difficult to get specimens of it, even a few grains in weight.

The mineral is not easily reduced before the blow-pipe, but is very easily reduced in a crucible with powdered charcoal or cyanide of potas sium; giving, at a single operation, buttons of star antimony.