380 Mr. J. H. Cotterill on the Equilibrium of 



series, and in the paratartrates and bitartrates. Like the alka- 

 lies, it forms double salts, the number of which increases the 

 more this curious metal is studied. It forms neither subnitrate 

 nor subacetate; but its acetate, distilled with arsenious acid, fur- 

 nishes cacodyle. Lastly, thallium has, like the alkaline metals, 

 and them alone, the characteristic property of forming thallic 

 alcohols. 



M. Lorin* describes a mode of reduction which is an applica- 

 tion of the following property. An ammoniacal salt, a simple 

 or a compound base, gives in general, in the presence of zinc and 

 water, a disengagement of hydrogen — frequently at the ordinary 

 temperature, but better towards 40° and upwards. 



This property has been verified on about fifty ordinary salts of 

 ammonia, and on a small number of salts of methylamine, ethyl- 

 amine, aniline, and naphthylamine. 



The quantity of hydrogen appears to be a function of the 

 equivalent of the acid. An equivalent of sulphate of ammonia 

 gave at least an equivalent of hydrogen. 



Of the ordinary metals, iron is the only one which approaches 

 zinc in its action on ammoniacal salts, though it is less intense. 



The concurrence of zinc, iron, ammonia, and an ammoniacal 

 salt are the best conditions for accelerating the production of 

 hydrogen. The rapidity of the disengagement is almost equal 

 to that for dilute acids. 



One exception is met with in the case of nitrate of ammonia, 

 which on dilute solution gives protoxide of nitrogen at a tempe- 

 rature near 50°. 



LIV. On the Equilibrium of Arched Ribs of Uniform Section. By 

 James H. Cotterill, B.A., Scholar of St. John's College, 

 Cambridge-^. 



1 . fT^HE object of this article is to show how to find approxi- 

 -8- mately the thrust, shear, and bending moment on any 

 section of an arched rib acted on by given forces under given 

 circumstances ; the method is founded on the principle of Least 

 Action, which in a former article was employed in the case of 

 straight beams. 



Let transverse planes be drawn indefinitely near to each other, 

 meeting in the centre of curvature at any point, cutting off a 

 small portion of the rib. Let the forces which may be acting 

 on the rib, whether distributed or detached, be supposed to act 

 at points in the line joining the centres of gravity of its trans- 



* Comptes Rendus, April 10, 1865. 

 t Communicated by the Author. 



