THE ELECTRIC CONDUCTIVITY OF NITRIC ACID. 383 



(iii.) Cell for Acids from 70 to 99'8 per cent. Concentration. 



As the acid above 70 per cent, is very hygroscopic and its fumes are liable to 

 attack the electric connections, a cell for use not only at a particular but also at any 

 required temperature was devised, as follows : a cylinder of glass, with an etched 

 centimetre scale, was drawn out at its lower end and a vacuum tap sealed on ; the 

 upper end was closed by an accurately ground glass cap carrying a tube on which 

 were etched two lines at a distance of 10 '24 centims. apart. Within this tube moved 

 a piece of thermometer tubing so as to form a stuffing-box arrangement containing 

 within it the platinum lead from the movable electrode ; one line was etched upon 

 it, which could be made to coincide with either line upon the outer tube. The 

 method of measurement was similar to that described in (i.) (standard cell), and the 

 whole was, as in previous cases, thoroughly insulated. It was found that acid could 

 be preserved in this cell for a considerable time without any appreciable change of 

 concentration. 



The electrodes for all the above cells were coated with the velvet-like deposit 

 of platinum, which, according to the experience of KOHLRAUSCH and successive 

 writers, gives the most satisfactory results. After preparation they were kept 

 immersed for some weeks in the prepared distilled water, which was frequently 

 changed. 



TANKS FOR ELECTROLYTIC CELLS. 



Two zinc tanks, of many gallons capacity, and furnished with covers, were used in 

 the course of the investigation ; they were both similarly constructed, one of two, the 

 other of three concentric chambers ; the innermost, of small dimensions, containing the 

 electrolytic cell with its accessories, stood on three legs, so as to raise its level about 

 six inches ; the other, of large dimensions, furnished with ring stirrers, was filled with 

 water to a level above that of the top of the electrolytic cell. It was found that a 

 difference of 20 between the tank and room temperatures caused only a fall in the 

 former of 1 after two hours ; there was, therefore, no appreciable alteration of 

 temperature in the course of any set of experiments lasting only a few minutes. 

 Standardised thermometers were placed both in the outer and inner chambers ; 

 when their corrected readings differed only by O'l it was assumed that equilibrium of 

 temperature in the outer and inner chambers had been established. 



When a temperature of was required, the outer chamber of the triple-chambered 

 tank contained air only, and the middle chamber was filled with finely-powdered ice, 

 the water melting from which was continuously drained off. For other temperatures, 

 water was run into the outer chamber, and steam blown in, if necessary. 



