W. H. Weed — Formation of Siliceous Sinter. 851 



respectively. From this it follows nearly, that for glass at 

 100°, ^=8X10 7 ; at 200°, w = 8xl0 9 ; at 360°, n=16xl0 1 \ 

 Thus it is fair to conclude that at temperatures quite as low as 

 100° the spontaneous chemical action, i. e. the continuous re- 

 arrangement of the molecules of glass is a pronounced occur- 

 rence. 



The given value of the frequency of discharge of field, n, 

 may be further expressed in terms of the number of molecules 

 m, which break up per unit of volume, per unit of time, when 

 the number of molecules q per unit of surface, whose disinte- 

 gration just discharges the field, and the mean distance, a?, over 

 which they are urged by the field during the interval between 

 break-up and recombination, are known. For n=mx / q ; or 

 m=n (q /x). Here a? is a very small quantity, not exceeding 

 the centimeter numeric of the mean free path of the molecule 

 of a gas ; whereas q is a very large quantity. Hence in is larger 

 than the given value of n, even if the above superior limits 

 be 100 times the true value of n. 



These approximate statistics are the nearest exact statement 

 for the phenomenon of molecular break-up, which I can ad- 

 duce ; but they suffice for the present purposes. They show 

 that even when glass is practically an insulator, the number of 

 active molecules m, considered absolutely is very large ; and 

 that m need by no means be negligibly small even in com- 

 parison with the total number of molecules per unit of volume. 



The above paragraphs prove that the rate at which molecu- 

 lar break-up takes place is appreciably greater when glass is 

 under stress than when it is not. It is improbable that the 

 system will pass from one state of molecular equilibrium to 

 another, instantaneously. Hence, since even in case of very 

 high resistance, such as that of glass at 100°, the number of 

 unstable molecules per unit of volume must still be conceived 

 to be very large, it follows that the species of molecular break- 

 up in question may be looked upon as a fruitful cause of 

 viscous deformation.* 



Phys. Lab. U. S. G. S., Washington, D. 0. 



Art. XXXVII. — On the Formation of Siliceous Sinter by 

 the Vegetation of Thermal Springs ; by Walter Harvey 

 Weed, of the U. S. Geological Survey, f 



It is a well known fact that hot spring waters often contain 

 considerable silica in solution, particularly those issuing from 

 volcanic rocks, with which boiling springs are so frequently as- 



* This Journal, xxxvi, pp. 178, 179, 183, 202, 208. 1888. Phil. Mag., V, xxvi, 

 pp. 397, 398, 1888. 



\ Published by permission of the Director of the Survey. 



