138 F F. Nipher—The Isentropic Curve. 



in no case were the reddish deposits formed upon any clear 

 mineral of the first series described, and that in all cases where 

 mint nils of the first series were formed in cavities containing 

 the stratified deposits, they occurred in the same order and 

 manner as in any other cavity, being situated on the roof and 

 sides as well as upon the floor. Fissures filled with these red- 

 dish minerals were in fact seen in several cases leading into 

 cavities containing stratified deposits of the same minerals. 



In a third article, still other zeolitic species from Table 

 Mount;. in will be described. 



Art. XVII. - 



Votume and Temperature ;* by Francis E. Nipher. 



The equation of this thermodynamic surface is 



pv = RT, (l) 



where/?, v, T represent the pressure, volume and absolute tem- 

 perature, and where E is directly proportional to the volume of 

 a unit mass (or inversely proportional to the density) of the gas 

 at a standan pressure. 



- = A, -V = B, 



and (2) becomes 



dp = AdT - Bdv. (3) 



1°. To find the direction of maximum slope with respect to 



the v,T plane at any point on the surface. For this purpose 



pass a plane through any point in the surface, and at right 



angles to the v,T plane. Its trace upon the v,T plane is 



From (3) and (5) we have 



dp = (Aa - B) dv. 



* From Trans, of St. Louis Academy of Science, read April 3, 1882. 



