22 THE POPULAR SCIENCE MONTHLY. 



obtain the needed colors, and additions to the list are constantly- 

 being made as the result of further experimentation. Taking the 

 colors up in the order of the spectrum, the violet shades are gen- 

 erally produced from manganese or from very small quantities of 

 cobalt ; the deep blues, indigos, purple blues, and normal blues, 

 from varying proportions of cobalt ; peacock-blue from copper ; 

 the finest greens from copper and chromium ; and the dull sea- 

 water tint from ferrous oxide. The oxide of copper gives an 

 emerald green. The yellows come from a variety of sources : the 

 sesquioxide of uranium gives a fine fluorescent yellow ; the oxide 

 of lead a pale yellow, and the oxide of silver, applied as a pigment 

 to the surface of the glass, a permanent yellow stain. The higher 

 oxide of iron gives an orange color, but, as it has a strong tend- 

 ency to become reduced, it is necessary during the manipulation 

 of the glass to keep some oxidizing agent present, such as man- 

 ganic oxide. In the reds a number of excellent shades are readily 

 obtainable. Manganese furnishes a variety of pinkish reds and 

 pinks ; copper, in its lower oxide, the fine blood-red of Bohemian 

 glass ; and gold, the deepest and most brilliant of all reds, the well- 

 known ruby glass. This list, however, is but a fragment. It bears 

 to the complete array of color at the command of the glass-worker 

 much the same relation that an inventory of crude pigments would 

 bear to the fine distinctions housed in an artist's color-box. It is 

 only intended to give some little idea of the mineral bases utilized 

 for their color effect. The fine gradations of color, and the rich 

 and delicate tones, are the result of no such elementary chromat- 

 ics. Many substances have joined their forces to produce these fine 

 results. In many cases they have been obtained only after long 

 experimentation, and have a corresponding value in the eyes of 

 their discoverers. The magnificent window designed by Mr. John 

 La Farge, which now faces the chancel in Trinity Church, Boston, 

 owes the brilliancy of its peacock hues to the combined forces of 

 some seventeen ingredients. This is an extreme instance of com- 

 plexity, but it fairly represents the present tendency to secure a 

 multitude of effects even at the expenditure of a multitude of 

 agents. 



In addition to these metallic compounds a number of other 

 substances are used to produce either colors or unique effect. A 

 little carbonaceous matter yields an amber tint of very agreeable 

 hue, while the opalescence now so much in vogue and so justly 

 admired results from the presence of oxide of tin, arsenic, or lime, 

 or from native minerals, such as fluorite or the cryolite now im- 

 ported in such large quantities from Greenland. The superiority 

 of American art-work in glass is largely due to the introduction 

 of this effective opalescent glass. It was first used in this country 

 about ten years ago, by Mr. John La Farge and Mr. Louis C. 



