On Artificial Slone, 153 



The stalactite is a concretion composed of successive 

 layers of a circular or undulated form, which is the effect 

 of desiccation. 



The i?icrustation is a concretion in the form of a crust 

 applied to the surface or to the interior of a body. To 

 this we may refer the geode, which is a concretion in the 

 formof an envelope, spherical or nearly so, sometimes empty 

 and sometimes containing a nucleus. 



The pseiidomorphQsis is a concretion endowed with a 

 form foreign to its substance, and for which it is indebted 

 to its molecules filling a space formerly occupied by a body 

 of the same form. 



[To be continued.] 



XXII. On a Hard Artificial Stone that generates a consider 

 rahle Quantity of Heat during its Consolidation; with the 

 Application (if this Fact to the Cause of Volcanic Fires*-, 



X HE artificial stone about to be described presents a re- 

 markable example of the great degree of solidity which 

 water, in certain combinations, can acquire. 



Water forms more than half the weight in the composi- 

 tion of these stones : the other ingredients are one part of 

 sulphuric acid, and two parts of burnt clay (bricks or 

 earthen-ware) reduced to powder. 



The mixture of these substances yields, in fact, a solutioa 

 of sulphate of alumine : but when in the mixture circum- 

 stances are favourable to their reciprocal action, heat is 

 speedily produced, and the quantity evolved is sometimes 

 so considerable as to produce ignition in the mass. 



If from 25 to 30 hundred weight of materials be em- 

 ployed, this extraordinary phaenomenon lasts for more than 

 an hour. 



The most remarkable circumstance is, that if no water 

 be added to the mixture, when the reaction of the sub- 

 stances upon ench other is the strongest, themas$, although 

 still liquid, suddenly acquires a great degree of solidity: 

 the heat which jt produces is augmented, and the substance 

 afterwards becomes almost entirely insoluble. 



This last property being acquired by a mixture calculated 

 to yield ver) soluble salts, |)roves that there is a great pene- 

 tration of the earth by the water and acid, as the whole 

 jiiass forms a stony composition only. 



• From M. dc la Mcthc-ric's Journal df. Physuiuc. 



Tht 



