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Physical Properties of AmmoniaDensity
The earlier determinations gave rather low results. A more exact value, 0.5967 (air=1), found by Biot and Arago, was afterwards corrected to 0.5971. The weight of 1 litre of air at N.T.P. varies between 1.2927 and 1.2933 grams. The corresponding mean weight of a "normal litre" of ammonia is 0.7708 grams. Since the density of air relatively to hydrogen is 14.44, that of ammonia is 8.624, and the calculated molar weight is 17.4 (O_{2}=32), which is greater than the true molar weight, 17.032, on account of the low value of the product pv for ammonia at N.T.P. As in the case of other easily condensible gases, the compressibility at ordinary temperatures and moderate pressures is greater than that which corresponds to Boyle's Law. The product pv increases with diminution of pressure; if that at p=1 atmosphere is taken as 1 (at 10° C.), at a 1/2 atmosphere it becomes 1.00632. Hence the value of a, as given by the equation
is 0.01264 or 0.01257. The coefficients "a" and "b" of van der Waal's equation, which express the deviation from the gas laws in terms of a coefficient of molecular attraction and of the volume occupied by the molecules respectively, are 13,800 and 2.11. Thermal Expansion
The coefficients of expansion "a" are as follows over the temperature intervals specified:
Refractivity of Ammonia
The refractive index, n_{D}, for the sodium line is 1.000373 according to Lorenz, and 1.000379 according to Mascart. Values have also been obtained at different wavelengths λ as follows:
The spectrum of ammonia consists of a bright central maximum at λ=336μμ, a secondary maximum at 337.1, and a number of lines arranged in groups of three, which extend to a considerable distance in either direction. The spectrum is practically identical with the P group of the solar spectrum; hence the presence of ammonia in the absorbing atmosphere of the sun is definitely established. 
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