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The correspondence between the calculated and measured (X-ray) lattice parameters of the orthorhombic cell is very good for the a and c axes and fair for the b axis. The comparison does not take into consideration any lattice readjustment, such as homogeneous contraction (or expansion) parallel to certain crystallographic directions, which might take place during transformation. Thus, a homogeneous contraction by 0.015kX. along the b axis of the orthorhombic cell will bring almost exact agreement between the calculated and measured lattice parameters of the orthorhombic phase. Laurie acid (form C or a), C1~H240~, is monoclinic with a = 9.524, b = 4.965, c = 35.39 A.,/? = 129 ° 13'. The cell contains four molecules; density observed 1.032, calculated 1.034 g.cm. -3. The space group is C~-P21/a. Fourier projections were prepared and atomic co-ordinates determined. The angle of tilt T = 54 ° 52'.

one cycle is required even when the changes in coordinates are within their permitted range of exploration. The method does, however, converge rapidly with both centrosymmetric and non-centrosymmetric projections. In non-orthogonal projections there will be some interaction between the two positional parameters. This should not be serious provided the interaxial angle is not too different from 90 ° . The method has not yet been applied to such a projection so that the actual effect cannot be assessed.The structures so far refined have not involved large variations in the individual temperature factors but in all the cases where the new method has been used the greater part of the refinement of the temperature factor refinement has been completed in a single cycle, provided the overall temperature factor and the scaling factor were correct. It might be expected that the new method would deal more easily with large variations than the existing leastsquares methods, which tend to overestimate the reductions and underestimate the increases in temperature factors and necessarily need more than one cycle. Provided the coordinate refinement is well advanced there should be no problem of missing the 'hole'.There is no reason why the methods could not be extended to deal with three-dimensional data and anisotropic temperature coefficients except that the existing methods may well be adequate.It is considered that the methods will be most useful in two-dimensional work where there is a great deal of overlap which makes it very difficult to get the structure into the right 'hole' to start with.We are grateful to Prof. H. Form A 1 of laurie acid, C12tt2402, iS trlcllnlc, with a = 7"45, b = 5.40, c = 17.47 /~; c~ = 960 53', /~= 113 ° 8', and 7=81 ° 7'. The space group is Pi. The crystal is built of laurie acid dimers, which are packed together with the planes of all hydrocarbon chains parallel to each other (triclinie subcell), and with the carboxyl groups of one dimer adjacent to methyl groups of neighbouring dimers. The angle of tilt is 67 ° 20'.

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