Chemistry and Physics, 



405 



1. Reduction of aluminum from cryolite. — Wohler lias found it ad- 

 vantageous in the preparation of aluminum to mix the finely pulverized 

 and well dried cryolite with an equal weight of a mixture of 7 parts of 

 chlorid of sodium and 9 of chlorid of potassium previously melted to- 

 gether and then finely pulverized. The mixture is to be introduced into 

 a hessian crucible in alternate layers with slices of sodium, the separate 

 layers being pressed strongly together. For every 50 grammes of the 

 mixture, 8 or 10 grammes of sodium are to be used. The crucible must 

 be previously strongly dried. It is then to be quickly heated to a white 

 heat in a good furnace. At the moment of reduction a noise is heard 

 and some sodium is volatilized which burns with flame. After this the 

 heat must be kept up for a quarter of an hour to fuse the mass com- 

 pletely, and then allowed to cool. On breaking the crucible, the aluminum 

 is found as a single white regulus, usually with a crystalline surface. In 

 this way about 4 per cent of the weight of the cryolite is obtained, which 

 is only about one-third of the aluminum in the mineral. The aluminum 

 is free from silicon. — Ann. der Ch. und Pharm. xcix, 255, Aug. 1856. 



8. Researches on the Fluorids. — Fremy has communicated the results 

 of an elaborate investigation of the compounds of fluorine beginning as 

 it were, de novo, from the very elements of the subject, and re-examining 

 many points which have long been considered as settled. The author 

 sums up his conclusions in the following words : — 



(1.) Fluohydric acid may be obtained from anhydrous acid in a state 

 of purity by calcining, in a platinum apparatus, the fluohydrate of fluorid 

 of potassium, previously dried. In this state the acid is gaseous at ordinary 

 temperatures ; it attacks glass and all silicious substances strongly, con- 

 trary to the assertions which have been made on this point of late years. 



(2.) All the experiments described in this memoir, confirm the views 

 of the constitution of fluohydric acid now received by all chemists, and 

 shew that this acid really behaves like a hydracid. 



(3.) It results from the general study of the fluorids which has been 

 made, that these compounds may be divided into three classes, and that 

 to each of these classes belongs an assemblage of important properties. 

 The first class comprises the anhydrous fluorids which are comparable to 

 the chlorids ; the second the hydrated fluorids which behave, in all their 

 reactions, like fluohydrates ; in the third class we find the fluohydrates 

 of fluorids which are true acid salts. 



(4.) The anhydrous fluorids are remarkable for their stability; the 

 hydrated fluorids are, on the contrary, unstable, and sometimes decompose 

 even when they are dried in vacuo, disengaging fluohydric acid and 

 leaving as a residue an oxyfluorid or oxyd. 



(4.) The fluorids have a great tendency to unite to form double fluorids ; 

 this property belongs even to the insoluble fluorids. Thus these last 

 compounds must never be prepared by double decomposition, because 

 they always retain, in their state of double salt, a part of the soluble 

 salt which has been employed in their preparation. 



(6.) Hydrogen does not decompose all the fluorids by the aid of heat ; 

 thus it does not act on the fluorids of calcium : but it reduces with the 

 greatest facility, the fluorids of lead, tin, &c. The reduction of the metal- 

 lic fluorids by hydrogen, like those of lead and tin, which resist the action 



