Mr. Crum 071 the Acetates of Alumina. 307 



A boiling solutioa of potash or soda dissolves the eoagalum, and at 

 the same time changes it into the ordinary alumina, which is thrown 

 down in the state of terhydrate when the alkali is saturated by an 

 acid. 



Oil of vitriol also dissolves the solid part of the coagulum, and the 

 same substance dried ; especially when assisted by heat. Strong muri- 

 atic acid, at a boiling heat, does the same, though witli greater difficulty ; 

 and the products are the ordinary sulphate and muriate of alumina. 



Large quantities of the acetic salts may be added before they coagu- 

 late the aluminous solution.* When the solid part of the coagulum 

 produced by a strong solution of acetate of soda was afterwards freed 

 from that salt by pressure, it re-dissolved in pure water, and the solu- 

 tion was again coagulated by a fresh addition of the salt. An experi- 

 ment with acetate of lime gave the same result. 



The nitrates and chlorides coagulate also with great difficulty. 



Solutions of sulphate of soda, magnesia, and lime coagulate as readily 

 as a liquid containing the same quantity of sulphuric acid in the free 

 state. On examining one of these mixtures, the sulphuric acid was found 

 in the solid part of the coagulum, as before ; and the mixture remained 

 neutral. 



A small spoonful taken into the mouth becomes immediately solid from 

 the effect of the saliva. 



The digested solution of alumina which has not been deprived of its 

 acetic acid by boiling, requires about twice as much sulphuric acid to 

 coagulate it as does the boiled solution. 



One of the most characteristic properties of the digested and altered 

 acetate of alumina is its loss of the power of acting as a mordant. The 

 ordinary acetate, as is well known, forms a yellow opaque precipitate with 

 decoction of quercitron. That which has been thoroughly digested is 

 merely coagulated by that decoction, — the colour of which is but little 

 altered, and the coagulum is translucent. The same effect is produced 

 with decoctions of logwood, brazil-wood, &c. 



A quantity of the solution of hydrate of alumina was evaporated to 

 dryness at the heat of boiling water. After being pulverised and again 

 submitted to the same heat, it was moistened with sulphuric acid as 

 before described, and heated to whiteness. It lost 25-67 per cent, of its 

 weight. 



Hydrate from the Insoluble Binacetate. — It has been already mentioned 

 that when the precipitated binacetate is kept for an hour or two in 

 200 parts of boiling water, it is changed into the soluble binacetate. 

 It must be constantly agitated during that time. The substance so pro- 

 duced may be converted, like tlie original solution into the peculiar 

 bibydrate. Thirty to thirty-six hours digestion is sufficient to comjilete 



• llence, in preparing tlie binacetate of alumina which is to be used in obtaining the 

 biliydrate, it is better to employ an excess of acetate of baryta, than to leave iu the 

 Kolntion the sliglitest trace of sulphuric acid. 



