Royal Institution of Great Britain. 549 



between them and ammonia; it exists in them in ternary com- 

 bination with hydrogen and carbon ; oxygen was found in most 

 of them, but is apparently absent in cinchonia. In consequence, 

 however, of the imperfection of their ultimate analysis, no general 

 conclusions could be satisfactorily drawn respecting their atomic 

 constitution ; some peculiar form, however, of hydrocarbon ap- 

 peared essential to their saturating power in respect to acids, and 

 is, perhaps, connected with their high equivalent numbers. The 

 properties of salicine and of a new crystallisable principle from 

 elaterium were then shown, and the absence of nitrogen in those 

 compounds pointed out : its presence was, however, shown in 

 narcotine and in caffeine, bodies possessing many of the characters 

 of the former, and yet not salifiable. 



A magnificent collection of volcanic specimens, perfect in its 

 kind, collected from Vesuvius under the superintendence of Monti- 

 celli, and presented to the Institution by William Pole, Esq., 

 M.R.I., was laid upon the library tables. 



Several instances of thick-rolled lead, perforated by the larvae of 

 insects, were also exhibited. 



Feb. llth, 1831. Mr. Harris on the power of various substances 

 to intercept magnetic action. The recent discoveries in this depart- 

 ment of science go far to prove that every known substance is in a 

 greater or lesser degree open to magnetic excitation ; but it had not 

 yet been shown that non-ferruginous masses could screen or stop out 

 the action ; on the contrary, from the few experiments hitherto tried, 

 it was rather to be inferred that such masses were devoid of this 

 screening power.* In the course of an extensive inquiry, however, 

 by Mr. Harris, lately communicated to the Royal Society, it was 

 observed, that though a single plate of iron of about the tenth of an 

 inch in thickness, could effectually arrest the action of a revolving 

 magnet on a disc of copper, yet it had not the same effect on a disc 

 of iron. In the latter case it was found requisite to multiply the 

 intervening mass very considerably. Hence, it seemed reasonable 

 to infer that a screening power might actually be obtained by other 

 substances not containing iron, but which were susceptible of mag- 

 netic change, provided such substances were employed in large 

 masses : such was found to be the case. The apparatus employed 

 by Mr. Harris, and described by him, consisted of a magnetic disc, 

 delicately balanced by means of a ring of lead upon a fine centre, 

 and which was set rotating without sensible vibration, at the rate 

 of 600 revolutions in a minute, by means of a train of wheels and 

 a long silk line rapidly run off from its circumference. When the 

 disc was free of the silk and wheels, it was carefully covered by a 

 closed cylinder of glass, having aflat surface above and a light disc 

 of tinned iron moveable on a delicate point placed immediately over 



* See the interesting researches of Mr. Herschel, Mr. Babbage, and others, 

 detailed in the Philosophical Transactions. 

 VOL. I. MAY, 1831. 2 O 



