PLASTIC CLAY FORMATION. 259 



coloured by an intermixture of yellow clay. It is dull and opaqvie, with 

 an earthy fracture, and yields to the knife. It is infusible at 166° of 

 Wedgwood*, but fuses rapidly when exposed to the stream of the hydro- 

 oxygen blow-pipe : the result is a pearl white translucent enamel, a partial 

 combustion taking place during its fusion f. According to Stromeyer it 

 consists of 



Alumine, - 30 

 Sulphuric acid, 24 

 Water, - 45^ 



Crystals of gypsum are frequently disseminated through the masses of 

 alumine, and the two substances enter into various states of combination, 

 sometimes giving rise to specimens that are semitranslucent. Chalk flints, 

 indurated ochraceous clay, and other extraneous bodies, are also occa- 

 sionally enveloped. 



From the recent experiments of Dr. Clarke, it appears that the purer 

 masses of alurainite are destitute of sulphuric acid, and consist simply of 

 water and aluminous earth. Hence a suspicion has arisen that the sul- 

 phuric acid in the examples analyzed by Stromeyer, and Dr. WoUaston, 

 may have originated from the presence of gypsum ; this, however, is not 

 the case, since in many specimens it is evident that the sulphate of lime 

 has been decomposed, and the sulphuric acid entering into combination 

 with the alumine, has formed a true subsulphate. 



The hydrate occurs in friable masses, of the colour and consistence of 

 magnesia : it adheres to the tongue, and may be reduced to powder between 

 the fingers. In this respect it differs from the subsulphate, which possesses 

 considerable hardness, and is susceptible of a fine polish §. 



* Kir'wan. 



t History of the Gass Blow-pipe, ly E. D. Clarke, LL.D. 8vo. 1819, p. 56. The ex- 

 periments of my brother gave similar results. 



J Phillips' Mineralogy, 2d edition, 1819. p. 111. 



§ In the elegant compendium of geology, inserted in Professor Brand's Manual of Che- 

 mistry, are the following observations on this subject: 



" In the cUfFs at Newhaven, on the Sussex coast, a very curious series of changes is going 

 on. A stratum of marl, containing decomposing pyrites, hes upon the chalk, which gives rise 



LL 2 



