418 Miscellanies . 



25. Alum in Mica Slate. — This fact is of not unfrequent occur- 

 rence in this country. A crumbling, half decomposed mica slate, 

 now lies before us, containing beautiful incrustations and masses of 

 plumose alum in fine silky crystals not unlike those of the Island of 

 Milo, (Greece.) We are informed that the rock is found about three 

 feet from the surface of the earth, and the alum appears to be purer, 

 the deeper the digging proceeds. 



Some mica slates contain sufficient sulphur to burn blue when laid 

 on a hot shovel,* or coals, and the alum would appear to be formed 

 between the sulphur, becoming sulphuric acid, by the action of the 

 oxygen of air or water, or both, and the alumina of the mica, aided 

 probably by alkali in the same mineral. 



If materials of this kind should be found in abundance, they might 

 form the basis of a profitable manufacture. 



In the present instance, reference may be had to Mr. Christopher 

 Johnson, of Colchester, Conn., where the rock is found. 



Addition to Prof. Johnson's piece on sieain.j- 



On page 310, after "directratioof the density," line 7th, insert the 

 following : — It is true that if only one boiler in a range were to be- 

 come empty and exposed to excessive heat, at the same time, the 

 quantity of steam just calculated, would be, in part, distributed, 

 through the connecting pipe, to the others, at the moment of its pro- 

 duction, which would diminish in a measure the pressure in the 

 over heated boiler. It may be said on the other hand, that the over 

 heating of the outer shell will never be confined to the lower arch, 

 nor to a single boiler in a range ; and it is evident that the lower 

 boilers in a boat must in the cases supposed want steam room in pro- 

 portion as the upper want water ; and that the connecting pipe could 

 not, as generally constructed, convey away the steam so fast as it 

 would be produced. The boiler which had been most remote from 

 the wharf, has generally sustained the injury, in explosions that have 

 occurred immediately after putting off. 



* In this instance, heat exhales the smell of sulphur, hut without flame ; most of 

 the sulphur has evidently been acidified, 

 i Received too late for insertion in its proper place. 



