96 



the author has shown, suggests a method of determining the 

 molecular arrangement of an alloy ; and, in general, whether 

 any alloy he a chemical compound or a mixture. 



The author also enters into several details as to peculiar, 

 and, in some cases, singular reaction of these and other 

 alloys upon solutions of the salts of their own metals: thus, 

 certain alloys of lead and zinc decompose solutions of lead 

 as rapidly as pure zinc ; while others, containing much zinc, 

 act as lead towards the salts of lead. 



In the case of three metals, A , B, C, whereof A is £ + , and 

 C is£ —to B, the author investigates the question as to what 

 will be the electro-chemical relation of the atomic alloys of 

 A + C towards B, in solvent menstrua; and in the class 

 of alloys of copper and zinc, has determined the alloy of no 

 action, with reference to iron ; and has also found alloys 

 which protect iron in solvents electro-chemically as fully as 

 pure zinc, and yet are not themselves acted on by the sol- 

 vent. 



He enters into the subject of the specific gravities of the 

 alloys of Zn + Cu and Sn + Cu minutely, and shows reason 

 to doubt the accuracy of the published specific gravities of 

 most alloys of these and some other classes. 



Professor Mac CuUagh read a supplement to his paper 

 " On the dynamical Theory of Crystalline Reflexion and Re- 

 fraction." 



In his former paper on that subject (see Proceedings, 

 9th December, 1839) the author had given the general prin- 

 ciples for solving all questions relative to the propagation of 

 light in a given medium, or its reflexion and refraction at 

 the separating surface of two media ; but he had applied 

 them only to the common case of waves, which suffer no 

 diminution of intensity in their progress, and in which the 

 vibration may be represented by the sine or cosine of an arc 

 multiplied by a constant quantity. Some months after that 



