176 
Proceedings of the Royal Society 
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°, 5|-°, 12°, 22-i-°, 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 Yolatile 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, 
Ore H27i_j_i, H, 
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 
surface of the liquid in the casks in which it is imported into this 
country and the hydrocarbons w^ere shown by eudiometrical analysis 
to have the composition of a mixture in nearly equal proportions 
of the hydrides of ethyl and propyl. 
The first portions of incondensible gas evolved on warming the 
most volatile product of the distillation of petroleum on a manufac- 
turing scale were also found to contain a mixture of these hydrides, 
while portions of gas collected at a later period of the operation 
approached more closely to the composition of pure hydride of pro- 
pyl, or were mixtures of the hydrides of propyl and butyl ; -the last 
gas collected being nearly pure hydride of butyl. 
