1824.] Royal Society. 233 



diminished by hammering, if held in the air unsupported, or 

 rested upon any body not equally magnetic ; and the power of 

 very weak magnets, or bars with little or no magnetism, is 

 increased, if held upon any substance that is magnetic. In all 

 cases and circumstances, the hammering tends to bring the sub- 

 stances in contact into a similar state, the weaker being 

 strengthened, and the stronger weakened. 



A paper was also read, entitled " Observations on the Iguana 

 tuberculata, the common Guana." By the Rev. Lansdown 

 Guilding, BA. FLS. Communicated by Sir E. Home, Bart. 

 VPRS. 



This paper was very short : it commenced with some general 

 remarks on the necessity, in zoology, of describing animals from 

 living specimens, and on the errors which had been committed by 

 naturalists in stating the characters of certain lizards, in conse- 

 quence of inattention to that circumstance ; thus the gular pro- 

 cess of the lizards alluded to, had been erroneously described as a 

 pouch capable of dilatation. Mr. Guilding then proceeded to 

 describe briefly an organ on the parietal bones of the head of the 

 guana, to which he gave the name of foramen Homiamim, in 

 honour of Sir E. Home. 



Feb. 5. — A paper was communicated, entitled, " A finite and 

 exact Expression for the Refraction of an Atmosphere nearly 

 resembling that of the Earth." By Thomas Young;, MD. For. 

 Sec.RS. J 



The reading was commenced of the Bakerian Lecture, by 

 J. F. W. Herschel, Esq. FRS. 



Feb. 12. — The Bakerian Lecture was concluded. 



The subject of this Lecture is the phenomena exhibited by 

 mercury, and other fluid metals, when placed within the in- 

 fluence of an electric current transmitted through conducting 

 liquids. 



If a quantity of mercury be placed in a dish and covered 

 with a conducting liquid, through which an electric current is 

 transmitted from a voltaic pile of moderate energy, by wires not 

 in contact with the mercury, this metal will be thrown into a 

 state of circulation, the force and direction of which varies 

 with the nature of the liquid, the intensity of the electric 

 power used, and other adventitious circumstances. If the liquid 

 be sulphuric, phosphoric, or any of the more concentrated acids, 

 the circulation is excessively violent, even with weak electric 

 powers, and takes place in a directiony)w» the negative to the 

 positive wire. On the other hand, under alkaline solutions, 

 pure mercury remains at perfect rest in like circumstances ; 

 but if the least atom of potassium, sodium, zinc, or any metal 

 more electro-positive than mercury, be added to it, a violent 

 rotation is immediately produced, in the opposite direction, 

 or from the positive wire. From some trials, Mr. Herschel is led 



