LEWIS. — AUTOCATALYTIC DECOMPOSITION OF SILVER OXIDK. 721 



The stirrer was made very simply without the use of solder, combining 

 the ordinary method of stirring with the centrifugal, and it was so effect- 

 ive that the different parts of the bath never differed by more than oue 

 or two hundredths of a degree. 



The temperature regulator was simply a large mercury thermometer 

 made with a spiral of thin glass sealed to a vertical capillary (C). Into 

 this capillary two fine platinum wires were sealed, so that when the ex- 

 pansion of the mercury brought it into contact with the upper wire an 

 electric current from a storage battery was made, which, passing through 

 a telegraph instrument, shut off" a thin rubber tube supplying gas to one 

 of the burners. 



This regulator was, in fact, the only part of the apparatus that caused 

 the slightest inconvenience. The spiral being made of thin glass and 

 holding nearly a kilogram of mercury was very fragile, and, being weak- 

 ened further by the continued high temperature, bi'oke several times, and 

 had to be replaced. If a regulator of the same type could be made of some 

 suitable metal, which would also have the advantage because of its high 

 conductivity of responding more rapidly to changes in the temperature, a 

 very great constancy might be attained, for a thermostat working at this 

 temperature has two great advantages over the common water thermostat : 

 it is influenced very slightly by changes in the room temperature, and 

 there is no loss of heat by evaporation. It must, however, like all ther- 

 mostats, be protected from drafts of air. 



A mixture of potassium and sodium nitrates proved to be an ideal bath 

 li([uid. When pure, the mixture is as clear as water and as mobile. 

 Well-annealed glass apparatus and thermometers may be plunged directly 

 into the liquid again and again without breaking. 



The vessel of enamelled ware seemed to suffer no deterioration what- 

 ever, even after several months of contact with the molten salt. 



The apparatus for containing the silver oxide and measuring its rate 

 of decomposition is also sketched in Figure 1. The glass tube (D), about 

 one meter long, was filled with carefully dried silver oxide to a height 

 of about 10 cm., and the powder was shaken compactly into the end. 

 This was covered with a thin layer of glass wool (E) and the remainder 

 of the tube was filled with a loose-fitting glass rod (F), the object of which 

 was merely to diminish as far as possible all air space subjected to 

 changes of temperature, and thus to avoid fluctuations in volume. The 

 end of the tube containing the oxide was immersed deeply in the ther- 

 mostat, and the other end was connected by rubber tubing to a long brass 

 tube (G), in passing through which the evolved oxygen came to the tem- 



VOL. XL. — 16 



