Messrs. F. and E. Rodgers on certain Metallic Cyanurets. 9 1 



In confirmation of my views, I beg leave to lay before 

 your readers the following principle, which I have had nu- 

 merous opportunities of confirming. If y, x be two quantities 

 such that y = <p x, and if 8 be an independent quantity which 

 comes into the investigation of the equation y = Qx of neces- 

 sity, although it does not appear in the resulting equation it- 

 self, then, notwithstanding the equation y = <$>x is true in 

 general, there may happen to be values of which cause 

 some of the steps of the demonstration to vanish or become 

 impossible; and in such cases the equation y = $x is not ne- 

 cessarily true, though it may happen to be so. This princi- 

 ple I have found to be most frequently necessary when f x 

 = constant. I shall add two instances. 



1st, If a plane be immersed in a fluid at an angle, 0, with 

 the surface, the coordinates of its centre of pressure are con- 

 stant, whatever be the inclination. But if the inclination be 

 diminished without limit, that is, when the value of is 0°, 

 the coordinates of the centre of pressure are no longer of the 

 same constant value as before, but become those of the centre 

 of gravity. If we inquire into the reason of this, we shall 

 find that although does not enter into the expressions for 

 the coordinates of the centre of pressure, yet it is a quantity 

 necessarily introduced into the demonstration; and when is 

 made equal to 0, some of the steps of the investigation become 

 nugatory. 



2nd, The equation d a 2 V + d b 2 V + d c 2 V = for the attrac- 

 tion of a mass upon a particle, furnishes us with another in- 

 stance ; for although the left hand member is in general equal 

 to zero, yet when the attracted particle becomes one of the 

 particles of the attracting body, its value suddenly changes 

 from zero to — 4w; which circumstance will be found, as be- 

 fore, to arise from one of the steps of the demonstration be- 

 coming nugatory in the latter case. 



St. John's College, Cambridge, 

 Dec. 20, 1833. 



XIX. On certain Metallic Cyanurets. By Frederick and 

 Edward Rodgeus*. 



"DEFORE the description of the experiments relative to the 

 ■*-* alkaline cyanurets, it is necessary that the processes should 

 be considered by which these compounds have hitherto been 

 prepared. 



The process recommended by Berzelius for the preparation 



* Communicated by the Authors. 



N2 



