TECHNOLOGY. 595 



centage of the magnesium and lime silicates of the brick-clay 

 into sulphates. When the bricks become wet, these com- 

 pounds dissolve, and in dry weather succeeding storms, the 

 solution evaporating from the surface of the bricks leaves 

 them coated with these compounds, which have the prop- 

 erty of efflorescing in dry air. Analysis of this incrustation 

 showed it to consist mainly of sulphate of magnesia and lime, 

 thus confirming, so far as it goes, Mr. Trautwine's explana- 

 tion. Another source of the trouble (which in Philadelphia 

 and vicinity exists in aggravated form) is ascribed to the 

 formation of the efflorescing sulphate of magnesia by the 

 decomposition of mortar. Much, if not all, of the lime used in 

 the locality above named is burned from a magnesian lime- 

 stone (having almost the chemical composition of a dolomite), 

 and the result is a mixture of lime and magnesia, which, be- 

 ing very susceptible to the action of sulphurous vapors dif- 

 fused everywhere in cities by the burning of vast quantities 

 of coal, is gradually decomposed, yielding sulphate of lime 

 and of magnesia. The last-named salt being highly soluble in 

 water, it becomes, by diffusion and absorption in the neigh- 

 boring bricks, the chief cause of the defacement of the same, 

 the sulphate of lime being but slightly soluble. Mr. Pember- 

 ton, following after the above-named author, insists more 

 positively that the main cause of the trouble is in the mortar, 

 as just described, and that the formation of sulphates in the 

 bricks during their manufacture in a coal-burned kiln, while 

 it doubtless contributes something towards producing the 

 disfiguring incrustation, is responsible for only a trifling share 



of it. 



IRIDESCENT GLASS, 



said to be produced by the action of the vapors of chloride 

 of tin upon the finished wares at a high temperature, has be- 

 come a popular novelty in glass during the past year. The 

 iridescence, though decided enough to yield beautiful effects, 

 is not intense. More intense effects are produced with a 

 dark and more or less opaque body ; the mode of production 

 of the latter is not generally known. Attention has been 

 pointedly called to the disposition manifested by articles of 

 glass hardened by the La Bastie process to fly into fragments 

 spontaneously and violently. Siemens affirms that glass com- 

 pressed by his process is free from this disadvantage. The 



