276 SEC. 8. HEAT. 



1093b. Four Boxes, containing plates subjected to the action 

 of the above apparatus, and showing the isothermic curves pro- 

 duced by the fusion of the grease upon 



A. Bluish schist, from the valley of Sulvan, near Vernagaz. 



B. Coal schist, from neighbourhood of Motivon, near the Col 



de Voza. 



C. Another schist from the Col de Voza. 



D. Septypite from Tholy (Vosges). 



E. Clayey schist. 



F. Hyaline quartz, with faces cut parallel with the axis. 



G. Trooshte, silicate of zinc, with faces cut parallel with the 



axis. 



H. Gneiss, fine grained, from the Val Anzasca, Monte Rosa. 



M. Jannetaz. Paris. 



VIII. POLARISATION. 



1095. Forbes' Mica Plates for the polarisation of heat by 

 refraction. University of Edinburgh. 



The mica is split into numerous thin films by careful application of heat, 

 and fixed in the wooden tubes at the proper polarising angle. 



IX. MISCELLANEOUS. 



Lens of Benedetto Bregans, 42 cm. in diameter, given to 

 the Grand Duke Cosimo III. after 1690. With this lens Avesani 

 and Yargioni, 13 years after the extinction of the Accademia del 

 Cimento, made the well-known experiments on the combustion of 

 the diamond and other precious stones. Afterwards it served the 

 celebrated Sir Humphry Davy for his researches on the chemical 

 constitution of the diamond. 



The Royal Institute of" Studii Superiorly Florence. 



Plaster Model of Thermo dynamic Surface. 



Prof. J. Clerk Maxwell. 

 1O97. Apparatus employed by Professors Stewart and 



Tait for producing rapid Rotation in a Vacuum. 



Prof. Balfour Stcivart. 



The driving shaft is of iron, and passes into the receiver through an iron 

 tube containing mercury, which acts as a barometer. 



This instrument was constructed and in part devised by E. Beckley, of the 

 Kew Observatory, for an experiment conducted by B. Stewart and P. G. Tait. 

 The mechanical principle of its construction is as follows : The receiver in 

 which the rotation takes place is in fact the extended vacuum of a barometer. 

 Through the mercury of this barometer a shaft communicates a slow motion 

 to some machinery in the receiver. The velocity of this motion is there 

 greatly increased by means of a suitable train of wheels, until at length a 

 vertical disc is made to rotate with great rapidity on a horizontal axis. If 



