Mr. 0, D. Allen on Casium and Rubidium, 189 



bablynot be safe to theorize upon it till the facts relating to the 

 degree of visibility of the arch in different seasons of the year 

 have been ascertained by observations taken at the equator, in 

 positions, such as that of Quito, where the influences of atmo- 

 spheric changes are in a great degree eliminated. I will only 

 add one theoretical remark relative to the position of maximum 

 brightness, observed by Prof. Brorsen to be just opposite the 

 sun's place. It appears to me that the explanation of this sin- 

 gular fact must be wholly distinct from that of the luminous 

 arch. According to my theory of gravity-undulations, those 

 from the sun are propagated without impediment or retardation 

 through the body of the earth ; but it cannot be affirmed that 

 they undergo no modification whatever. It seems reasonable to 

 suppose that they partly consist of, or in their transmission give 

 rise to, a small amount of undulations that are subject in a 

 slight degree to retardation and consequent refraction, and that 

 to an effect of the convergence of these undulations the pheno- 

 menon in question may be attributed. In the " Theory of the 

 Force of Gravity" (Phil. Mag. for December, 1859), I have sug- 

 gested that a similar modification of terrestrial gravity-undula- 

 tions might account for the observed excess of gravity in insular 

 positions, and for deviations of the plumb-line noticed in India. 

 The concurrence and consentaneity of different parts of the 

 general physical theory in the foregoing explanations, embracing 

 both the zodiacal light and the associated phenomenon of the 

 luminous arch, ought, I think, to be regarded as some evidence 

 of the truth of the theory. 



Cambridge, February 20, 1863. 



XXV. Observations on Ccesium and Rubidium. By Oscar D. 

 Allen, Ph.B. } Assistant in the Sheffield Laboratory, Yale Col- 

 lege, U.S.* 



THE discovery of the presence of the new elements rubidium 

 and csesium in several varieties of European lepidolite, 

 made it a subject of interesting inquiry to ascertain whether 

 American lepidolite would not also serve as a source for these 

 rare metals. 



A preliminary experiment made last autumn by Mr. John M. 

 Blake and myself having shown that the lepidolite from Hebron 

 in Maine contains these alkalies in comparative abundance, I 

 was led to visit that locality, and there obtained the material 

 which served for the following investigation. 



* From Silliman's American Journal for November 1862. 



