Boyal Society of Edinburgh. 



319 



III. and lY.), the temperature of the night declined occasionally 

 to freezing. At Balfour it fell to 30°'0, and at the Botanic G-arden 

 (exposed) to 26°-0, — a degree of frost insufficient to damage those 

 autumn flowers which had stood the more severe frost in the begin- 

 ning of November, or check the growth of the spring flowers rapidly 

 coming into bloom. 



This anomalous weather sufficiently accounts for the strange 

 spectacle of siueet peas and Hepaticas blooming together. 



Monday, 15tJi February 1864.—Dr CHRISTISON, Y.P., 

 in the Chair. 



The following Communications were read : — 



1. On the Influence of the Eefracting Force of Calcareous 

 Spar on the Polarization, the Intensity, and the Colour of 

 the Light which it Eeflects. By Sir David Brewster, K.H., 

 F.E.S. 



In the " Philosophical Transactions " for 1819, the author had 

 shown that the doubly refracting force of calcareous spar extended 

 beyond the sphere of the reflecting force, producing a change in 

 the polarising angle varying with the inclination of the incident 

 ray to the axis of the crystal, and producing a deviation of the 

 plane of polarisation from the plane of incidence and reflexion, 

 when the reflecting force of the crystal was reduced by contact 

 with oil of cassia and other oils. These experiments were made on 

 the face of the primitive rhomb. 



In the present paper, the author gives an account of the results 

 which he obtained upon other natural and artificial faces of cal- 

 careous spar, inclined 0°, 51°, 12°, 22i°, 67|-°, and 90°, to the axis 

 of the crystal. On all these surfaces, when the reflecting force is 

 reduced by contact with oil of cassia or other oils and fluids, the 

 intensity and colour of the reflected pencil, and the deviation of 

 the plane of polarisation from the plane of reflection, experiences 

 remarkable changes, depending on the inclination of the incident 

 ray to the axis of double refraction. 



2. On the Most Volatile Constituents of American Petro- 



leum. By Edmund Eonalds, Ph.D. 



It was shown by this paper that the gases dissolved in American 

 petroleum, and which gave to it such a high degree of inflamma- 

 bility, were composed of the lower members of the marsh gas 

 series, having the general formula, 



and to which the liquid products have already been referred. 



The gases evolved from the Pennsylvanian oil were collected at a 

 temperature of - 1° Cent., as they floated, mixed with air, over the 



