284 M. Broncniart on the 



some detail, as I shall afterwards employ it as a type of com^ 

 parison with the same mineral, found in other positions and 

 in other places.* 



The magnesite of Coulommiers, in the purest specimens, 

 for it is often mixed with other things, possesses the follow-!- 

 ing characters : 



Its masses are soft, smooth to the touch without being 

 unctuous ; its powder is rather hard. 



It easily absorbs water and swells out considerably, be-r 

 comes slightly translucent, and forms a short soft paste, re- 

 sembling jelly. 



It does not effervesce with acids. 



Exposed to the action of a porcelain furnace (at 140° of 

 Wedgewood), it hardens, exfoliates a little, but does not 

 suffer any other alteration ; it does not shew the slightest 

 trace of fusion, either in its thin pieces or on the surface } 

 it however becomes rough to the touch, ^nd hard enough to 

 scratch steel. 



M. Berthier has analysed this magnesite, chosen from the 

 purest masses, and has found the following ingredients : 



Magnesia 24 



Silex 54 



Water 20 



Alumine 01,4 



99,4 



The magnesite of Coulommiers occurs in masses, which, 

 by their schistose structure and thinness, shew they belong 

 to thin beds. 



Its colour is whitish, most commonly pale grey ; it has 

 often a roseate tint, but it loses that and its grey colour in 

 the fire. It is associated with brownish and reddish chert 

 (silex come) of a very scaly fracture ; it is intimately united 

 with it, and penetrates into all its carities, and even into its 



* I am indebted to M. Merimee for the knowledge of this magnesite. 

 He was struck with tlie soapy unctuosity of a stone which he found at 

 Coulommiers, and having brouglit it to me, he put me in the way of dis- 

 covering this mincial in the Paiis basin. 



