Polytechnic Association. 755 



evaporated, and that the waters at a later period were interrupted by 

 a change in the configuration of the surface of the country. 



The presence of boracite leads us to suppose that the various bodies 

 of salts were deposited after their subjection to an internal heat, which 

 caused the diminished hydration mentioned above, the boracic acid 

 being introduced by the eruptive phenomena, whereby hydrochloric 

 acid may also have been generated, to which the chlorides may owe 

 their existence. It is well known that rock-salt is one of the products 

 of volcanic emanations and of springs in volcanic regions ; and it has 

 been shown, also, that salt may be traced to a certain depth associated 

 with lavas. 



Many brine springs rise through strata of sandstone and red marl. 

 "We find in England large beds of rock-salt, and brine springs which 

 have been flowing for 1,000 years. 



In the Triassic period we find salt, gypsum and magnesian limestone 

 more or less associated, while the gypsum and salt lie in many locali- 

 ties in the blue clay, without the red sandstone. After consideration 

 of the subject, I cannot think but that the origin of rock-salt is derived 

 from the evaporation of lakes and lagoons communicating with the 

 ocean. 



A salt lake on the Abyssinian frontier, exposed to the unmitigated 

 rays of the sun, is known to have been shrunk into an elliptical basin 

 seven miles in its transverse axis, which is half filled with water, and 

 the other half with a sheet of snow-white solid salt. 



The Dead Sea is known to contain pure salt in its water. 



In the United States, the rock-salt deposits of Louisiana are of 

 immense depth. In Nevada are salt deposits fourteen feet in thick- 

 ness and five miles square. The salt springs of Illinois pass through 

 five strata of coal, and then through the new red sandstone, and dis- 

 charge daily thousands of gallons. 



The President — During the war the southern portion of our country 

 was very much troubled to get saltpeter for the manufacture of gun- 

 powder. Nitrate of sodium exists in large quantities in this country 

 and in South America, and it seems, from the paper which has just 

 been read, that the chloride of potassium exists in large quantities in 

 Prussia. Now we only want these two substances, to produce the 

 nitrate of potassium or saltpeter, and the chloride of sodium or com- 

 mon salt. 



Etching on Glass. 



Dr. Feuchtwanger exhibited specimens of glass etched by Mr. C. 

 A. Stade with hydrofluoric acid, by a new process. 



