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The structure of nitroguanidine has been determined and refined by least-squares and Fourier methods. The molecule has the formula C(NH2)21N-NO2(I) ; the structure of the molecule is a resonance hybrid. The bond distances are discussed in terms of the various resonance forms for the molecule. The crystals are held together by a satisfactory system of hydrogen bonds.

The viscosity and density of the system TNT-picric acid and four related susbstances have been investigated over a range of temperature which included both normal and supercooled liquids. The temperaturedependence of viscosity was found to be best represented by a continuous exponential function of the type '1=A exp(B/T +C/P), which yielded a good fit to the data even well into the supercooled region. Density in each case was found to vary linearly with temperature across the entire range. The results are interpreted as arising from an inverse temperature dependence of the flow activation energy, AEvis. It is suggested that AEvis may drop to zero at sufficiently high temperature and pressure.

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method is shown in Table IV. Both methods are accurate to within about 1%. The proposed spectrophotometric method is much more rapid and convenient.Comparison of Thiocyanate and Spectrophotometric Methods. Mercury compounds may be determined by direct titration with standard thiocyanate (4)• Table V shows that for pure phenylmercuric acetate this method is as accurate as the proposed spectrophotometric method. It is also rapid and convenient. However, mercuric salts, such as mercuric acetate, interfere. This is particularly serious, as the thiocyanate and phenylmercuric acetate react in a 1 to 1 mole ratio, whereas thiocyanate and mercuric acetate react in a 2 to 1 ratio. As shown in Table II, 7.85 X 10 "6 mole of mercuric acetate in 100 ml. of solution containing 1.26 X 10""4 mole (0.0425 gram) of phenylmercuric acetate causes less than 2% interference in the spectrophotometric method; this concentration causes over 120% error in the thiocyanate method, as is shown in Table V. LITERATURE CITED(1) Beckman Instruments, Inc.. Los Angeles, Calif., (2) Gowenlock, B. G., Trotman, J., ./. Chem. Soc. (London) 1955, 1454