351 



experiments in which the electric spark was found to have penetrated 

 through the side of a glass globe blown to an extreme degree of thin- 

 ness. An electric jar, from which the air had been partially ex- 

 hausted, could not be made to receive as high a charge as when the 

 contained air was of the usual density, and when entirely exhausted 

 could not be charged in any sensible degree ; when filled with con- 

 densed air on the other hand, it retained a higher charge than before. 

 The heated and consequently rarefied air surrounding a red-hot iron 

 rod is found to conduct electricity with great facility. The same 

 property is observed in the flame from a blowpipe, which may be re- 

 garded as a hollow cone containing highly rarefied air ; as also, hi 

 a larger scale, in that of a volcano. Sir H. Davy had concluded from 

 his experiments on voltaic electricity, that the conducting powers of 

 metals are diminished by heat; but Mr. Ritchie infers from several 

 experiments which bear more directly upon the question, that the 

 metals afford no exception to the general law, that in all bodies heat 

 increases the conducting powers ; and explains the apparent anomaly 

 in Sir H. Davy's experiments, by the dissipation of the electricity by 

 the rarefied air surrounding the heated metals, which were used as 

 conductors. He concludes his paper by describing an experiment 

 which appears to establish, in respect to this law, a striking analogy 

 between the electric and magnetic influences. 



On Magnetic Influence in the Solar Rays. By Samuel Hunter Christie, 

 Esq. M.A. F.R.S. So. Read June 19, 1828. [PAi7. Trans. 1828, 

 p. 379.] 



From the experiments described by the author in a former paper, 

 it appeared that a magnetized needle vibrated under exposure to the 

 sun's rays, came to rest sooner than when screened from their in- 

 fluence ; that a similar effect was produced on a needle of glass or 

 of copper, but that the effect on the magnetized needle greatly ex- 

 ceeded that upon either of the others. In the prosecution of this in- 

 quiry, the author has endeavoured to vary the experiments so as to 

 obviate several causes of inaccuracy which might tend to invalidate 

 the general conclusions he had before drawn. His first object was 

 to compare the effects of the solar rays on an unmagnetized steel 

 needle with one that was magnetized under the same circumstances ; 

 and the result was, that the latter was influenced in a more consi- 

 derable degree than the former ; and a similar difference was ob- 

 served when the vibrations of a magnetized needle were compared 

 with those of a needle made of glass or of copper. He ascertained 

 that the diminution of the terminal arc of vibration, on exposure to 

 the sun, was not occasioned merely by the heat imparted to the 

 needles or surrounding medium, although this cause appeared in some 

 instances to measure the intensity of the action which produced the 

 diminution. In order to determine the comparative influence of the 

 separate rays, he allowed them to fall on the needles after transmis- 

 sion through differently coloured fluids and glasses ; but owing to 



