492 Mr. Lubbock on the Variation of the 



red-brown powder by trituration, and having exactly the com- 

 position of the two preceding salts. After this green crust 

 has been separated, crystals of chromate of silver and am- 

 monia of a pale yellow colour form (this salt has been carefully 

 examined by Professor Mitscherlich), and lastly bichromate 

 or chromate of ammonia separates. 



Should it be wished to obtain the double chromate of silver 

 and ammonia, care should be taken to exclude the air, as 

 the ammonia rapidly passes off and destroys the object in 

 view. I have found that when placed in a partial vacuum 

 over quicklime, an atmosphere of ammonia is formed around 

 the solution, while at the same time the lime is gradually 

 absorbing the watery vapour. 



In a future communication I hope to consider the action of 

 chromic acid upon some of the other metallic bodies, as well 

 as to recite an examination of a variety of double chromates 

 formed during that action. 



LXIX. On the Variation of the Arbitrary Constants in 

 Mechanical Problems, By J. W. Lubbock, Esq., F.R.S* 



HPHE general theory of the variation of the arbitrary con- 

 -*■ stants in mechanical problems, with reference to the dif- 

 ferential coefficients of the disturbing function considered as 

 a function of those coefficients, as hitherto exposed, appears 

 susceptible of several simplifications. 



Suppose the differential equations of any dynamical pro- 

 blem to be 



This general theory of the variation of the arbitrary con- 

 stants in mechanical problems may be said to consist in the 

 following theorems ; 



* Communicated by the Author. 



