The cartesian equation referred to the axis and directrix is y=c cosh (x/c) or y = Zc(e x / c +e x / c); other forms are s = c **sinh** (x/c) and y 2 =c 2 -1-s 2, being the arc measured from the vertex; the intrinsic equation is s = c tan The radius of curvature and normal are each equal to c sec t '.

Silicon nitrogen hydride, **SiNH**, is a white powder formed with silicon amide when ammonia gas (diluted with hydrogen) is brought into contact with the vapour of silicon chloroform at -10° C. Trianilino silicon hydride, SiH (NHC 6 H 5) 3, is obtained by the action of aniline on a benzene solution of silicon chloroform.

It is ~asily seen graphically, or from a table of hyperbolic tangents, that the equation u tanh u = 1 has only one positive root (u = 1.200); the span is therefore 2X =2au =2A/ **sinh** U = 1.326 A,

If A, B have opposite signs the form is au = **sinh** mO, (24) this has an asymptote parallel to 0=0, but the path near the origin has the same general form as in the case of (23).