Proceedings of the Polytechnic Association. 853 



wlieel and pawl of an index finger and plate, that the automatic open- 

 ing and closing of the circuit by the mechanism of the gauge, caused 

 • the index to indicate accurately the degree of pressure. A register- 

 ing thermometer, also inclosed within the shell, was used to ascertain 

 the temperature. This apparatus he tried in the bay of New London, 

 and it measured very accurately. We found sixty-seven feet of water 

 there, which surprised several captains of vessels to whom he men- 

 tioned the result. 



Prof J. A. Whitney said that an invention for deep sea sounding 

 was lately brought out in England, in which the temperature is ascer- 

 tained by the varying resistance of metals to electrical currents at 

 diiferent temperatures ; the increased resistance of the submerged 

 wires indicating a proportionate rise in the temperature, and the 

 greater resistance being made apparent through the agency of a suit- 

 able galvanometer. This appears to be a very accurate instrument. 



Anti-Friction Valve. 

 Mr. W. Thilmany exhibited an anti-friction balance valve. A brief 

 discussion ensued, during which Mr. C. E. Emery stated that the 

 device was not novel. 



Manufacture of Nitric Acid. 

 Dr. Isidor Walz explained his new method of manufacturing 

 nitric acid and soda from nitrate of soda or Chili saltpetre. It con- 

 sists in heating carbonate of lime and Chili nitre in retorts, into 

 which steam is at the same time admitted. The chemical reaction 

 taking place is expressed by the formula : Ca O, C 0,^+Na O, N 

 0^+H 0=Ca 0-f-Na O, C O^+N O^ H O. The nitric acid is not 

 split up, as it would be without the presence of steam, into oxygen and 

 hypo-nitric acid, entailing the loss of from one-third to one-half the 

 quantity present, but distills over pure, is condensed in Wolfe's bot- 

 tles, and afterward concentrated. In this way nearly the whole theo- 

 retical quantity can be recovered. A nitrated mass is left in the 

 retorts, consisting of caustic lime and carbonate of soda. This maj 

 be sold directly to manufacturers of soap and others, or it may be 

 worked up in the factory, yielding carbonate of soda when leached 

 with cold, and caustic soda when leached with hot water. The advan- 

 tages claimed are greater purity of the articles, especially of the acid, 

 simplicity of manipulations and processes involved, and cheapness. 



