The system MgO-Si02 has been studied at temperatures up to 1800". The polymorphic relationships in magnesium metasilicate are discussed and evidence is offered for the possible existence of an additional phase in the high-temperature region of the system. London, S.W.7THE MgO-SiO, system has frequently been investigated.Early work on the melt equilibrium of the system showed,l by optical methods, that magnesium orthosilicate (Mg,SiO,) (forsterite), and the clinoenstatite modification of magnesium metasilicate (MgSiO,) crystallised from the melt. The incongruent melting of clino-

The ionic and total fluoride levels in pasteurised and untreated milk have been examined using a fluoride sensor. Ionic fluoride was determined in potassium nitrate and sodium citrate buffer solutions and total fluoride was determined after pyrohydrolysis or perchloric acid hydrolysis. Binding of added ionic fluoride to milk-calcium or milk-protein occurs after addition, making milk an unsatisfactory carrier of ionic fluoride.

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Studies have been undertaken to determine the uptake and release of fluoride from hydroxyapatite and it has been concluded that the effect of a fluctuating pH on hydroxyapatite in the presence of a fluoride solution is to increase the fluoride content of the hydroxyapatite by a mechanism involving: (1) partial dissolution of hydroxyapatite at pH 4.0; (2) initial formation of calcium fluoride at pH 4.0 and (3) transformation of calcium fluoride into fluoroapatite as the pH increases. An attempt has been made to correlate these findings with other reports describing the incorporation of fluoride into mineralised hard tissues.

Complexes of benzothiazole (BT) have been prepared and studied by magnetic and spectroscopic techniques. Tetrahedral complexes, M(BT),X,, which have been well characterised, are formed with ZnX, (X = CI, Br) and COX, (X = CI, Br, I. NCS). Tetrahedral Ni(BT),I, has also been formed, but the following nickel complexes have a n octahedral or distorted octahedral stereochemistry : Ni(BT),Br,, Ni(BT),X, (X = CI, Br), Ni( BT)Br,. The Ni (BT) ,X2 species involve halogen-bridged polymeric structures with benzothiazole molecules above and below the plane, while in Ni(BT)Br, the planes of nickel atoms and bridging bromine atoms are connected by bridging benzothiazole molecules. It is suggested that bridging benzothazole is also present in [Cu,( BT),] Tetragonal polymeric copper(it) complexes, Cu(BT),X,, with bridging X (CI, Br) and 1 : 1 complexes, Cu(BT)X,, which it is suggested are dimeric, four-co-ordinate, with bridging X, are also reported. A number of other 1 : 2 species with CuBr, are reported, together with some octahedral perchlorates and perchlorato-complexes of Ni( ii) and Co( 11). It appears that benzothiazole co-ordinates through the nitrogen heteroatom in these complexes (except when acting as a bridging ligand, when both heteroatoms are involved).