Intelligence and Miscellaneous Articles. 235 



PHILOSOPHICAL SOCIETY OF CAMBRIDGE. 



May 20. — The following communications were read: — 



On the Attraction of Spheroids, by G. Green, Esq. In this paper 

 the author presents certain analytical formulas, in reference to triple 

 integrals of a more general form than those offered in the attractions 

 of spheroids of arbitrary form and density, and applies them to the 

 problem of the attractions of ellipsoids, so as to comprise the ac- 

 tions on points, internal and external, in a common process, by the 

 addition of a positive quantity, under the radical sign in the expres- 

 sion, for the reciprocal distance between the point acted on and any 

 point of the ellipsoid, which quantity is afterwards made to vanish. 



On the Determination of the Vibratory Motion of Elastic Fluids 

 in Tubes of definite Length, by W. Hopkins, Esq. of St. Peter's 

 College. The author described a series of experiments made by 

 him with the view of subjecting to an experimental test the dif- 

 ferent solutions which have been given of this problem. The in- 

 tensity of the vibrations in any part of the tube are indicated to 

 the eye by the motion which those vibrations excite in a delicate 

 membrane, sprinkled with light sand, and suspended in the tube. 

 The positions of the nodal points, thus determined with great ac- 

 curacy, are not such as accord with any solution of the problem 

 hitherto given ; but it was shown how all the observed phenomena 

 are accounted for by the assumption of certain physical conditions 

 more general than those assumed by previous writers. 



An experiment was also exhibited by Mr. Hopkins, showing the 

 effect of the interference of two aerial undulations proceeding in the 

 same direction. The ends of two equal tubes, branching off from one 

 common tube, are placed close to two ventral segments of a vibra- 

 ting plate, by which the vibrations are excited in the branch tubes 

 and interfere in the one with which they communicate. If the vi- 

 brations proceeding from the two ventral segments be in the same 

 phase, the resulting vibration is one of great intensity ; but if they 

 are in opposite phases no sensible vibration results from them. The 

 intensity of the vibration is indicated, as above mentioned, by a 

 membrane which may be stretched over the mouth of the tube. 



XLII. Intelligence and Miscellaneous Articles. 



COMPOUNDS OF CHROMIC ACID WITH METALLIC CHLORIDES. 



ME. PELTGOT has formed several compounds of chromic 

 • acid with metallic chlorides. The bichromate of chloride of 

 potassium is easily and ceconomically produced by boiling a mixed 

 solution of bichromate of potash and muriatic acid for some time; 

 on cooling, a quantity of the salt in question crystallizes, in pro- 

 portion to the quantities of ingredients employed. 



According to M. Peligot the theory of the formation of this salt is 

 as follows : the muriatic acid at first acts upon the potash of the bi- 

 chromate, and forms chloride of potassium, and water, and at the 



2H2 



