A12 Interpretation of Mariotte’s Law. 
The constitution of the mineral, so different from all known 
feldspars—its 8 per cent. of magnesia, 7 per cent. of oxyd of iron, 
and 7 percent. of water—entirely favor the opinion that the crys- 
tals are actually pseudomorphs of some feldspar mineral as Brei- 
thaupt suggested. Changes through pseudomorphism of the ex- 
tent this would imply, and even far greater, are abundantly exem- 
plified. 
Art. XLIIT—On the Interpretation of Mariotte’s Law ; by 
Lieut. E. B. Hunt, U. 8. Corps of Engineers. 
medium, the component parts of which act*on each other by 
forces varying as any function of the distance, Mariotte’s law 
must prevail. Both elastic tension and cohesive force will neces- 
sarily vary as the density, in a medium assumed as homogeneous, 
quite irrespective of the law of force, the variation being expressed 
in terms of distance between the component parts of the me- 
dium. Whether the force be attractive or repulsive, varying in- 
versely with the first or hundredth power of the distance, the 
result is the same; that entire homogeneousness makes Mariotte’s 
w necessary. i 
To prove this: assume a perfectly homogeneous medium whose 
parts exert forces varying as any function of the distance. As- 
sume in this an origin of coérdinates, three coordinate axes, X, Y 
and Z, and three constant elementary distances, dz, dy,dz. Con- 
ceive each axis graduated by laying off its element successively 
from the origin outward. Through each point of graduation on 
either axis pass a plane parallel to the other axes: do this for each 
axis. The space around the origin is thus divided into element- 
ary parallelopipeds, each of which contains a like portion of the 
omogeneous medium. 
The force of elastic tension or of cohesion is measured by the 
resultant action on a unit of surface of the plane X, Y, by all the 
forces acting in the positive direction of the axis Z, between the 
parts on opposite sides of the plane X, Y. This resultant is bal- 
ance by an equal one acting in the negative direction of the 
axis Z. To make up this resultant, a certain number of the ele- 
mentary portions of the medium conspire. It may therefore be 
equated with a series, each term of which expresses the positive 
component along the axis Z, of the force exerted between two 
elementary portions of the medium on opposite sides of the 
plane X, 
If now the density of the medium be varied, each term of this 
series will vary in the same ratio, since the quantity of matter in 
Ir is readily demonstrated that in any entirely homogeneous 
hich 
