340 Analysis of Books. 



extremity of the index from the shoulder of the black lead bar. The 

 spaces comprehended between the points of the shorter legs of the 

 compasses, are magnified ten times by the longer legs, the angular 

 motion being measured by a graduated arc furnished with a vernier, 

 and capable of being easily read oft' to minutes. 



The author next enters into a comparison of the results afforded 

 by this instrument with those of former experimentalists, and 

 especially with the accurate determination of the expansions of 

 metals by Messrs. Dulong and Petit, with a view to shew the 

 degree of confidence to which it is entitled. The close agreement 

 in the results of a great number of experiments upon metals, 

 which differ much in their expansions, is highly satisfactory in this 

 respect. Differences having been found in the expansibility of 

 different specimens of black lead, it becomes necessary to ascertain 

 the expansions of each register for itself, by applying to it the heat 

 of boiling mercury. 



The author concludes with an account of some experiments 

 which he made to determine the fusing points of different metals, 

 referred to the common thermometric scale. The final results 

 which he obtained were for silver, 1873; copper, 1996; gold, 

 2016; iron, 2786. 



A, remarkable accordance is found between the results with 

 platina and with iron, metals which differ widely in their expan- 

 sions ; conformably with the conclusion of MM. Dulong and Petit, 

 the expansion of iron increases at higher temperatures in a greater 

 ratio than that of the platina. The discrepancy between the tem- 

 peratures derived from the observations with his first pyrometer 

 and the present one he admits to be considerable, but believes they 

 may be sufficiently accounted for by the differences in the circum- 

 stances of the experiments, without imputing inaccuracy to either 

 instrument. 



The author next attempted to ascertain the effects of the most 

 intense heat which it was possible to produce in a furnace, and to 

 measure the utmost limits of expansion in a platina bar ; but various 

 circumstances interfered with the success of these experiments, 

 which afforded, however, many curious results as to changes 

 of integration in platina by the effects of heat. The paper con- 

 cludes with some observations on the practical advantages possessed 

 by the present instrument*. 



5. On the Phe?iomena and Laws of Elliptic Polarization, as exhi- 

 , bited in the Action of Metals upon Light. By David Brewster, 

 LL.D., RR.S. L. and E. [Read April 22, 1830.] 



THE action of metals upon light has always presented a remark- 

 able, and hitherto inexplicable anomaly in the science of polariza- 

 tion. Malus, to whom this branch of optics owes its origin, had at 

 first announced that metals exerted no polarizing influence on 



* The register-pyrometer is made by Mr. Newman, 122, Regent Street. 



