147 



out the principles indicated by Mr. Welsh, by carefully removing all 

 trace of moisture, and thoroughly cleaning the tubes before intro- 

 ducing the mercury, the author succeeded in obtaining Torricellian 

 vacuums which exhibit the stratifications in a uniform and very 

 marked manner. 



The sealed tubes generally used by Mr. Gassiot are then described. 

 They are made of the usual glass tubing, about an inch internal 

 diameter, and of the form fig. 1 . 



They vary from 10 to 38 inches in length. In the latter case the 

 platinum wires a b are about 32 inches apart. One tube is de- 

 scribed 5 feet 3 inches in length, with wires 4 feet 9 inches apart. 



With a tube prepared on Mr. Welsh's principle, and the usual- 

 sized Ruhmkorff 's induction-coil excited by a single cell of Grove's 

 nitric acid battery, with or without a condenser, the phenomena of 

 the stratified discharge can be seen and examined with ease, and 

 without the trouble and uncertain manipulation of an air-pump, or 

 the employment of phosphorous or other vapours. 



If the discharges are made in one direction, a black deposit takes 

 place on the sides of the tube nearest the negative terminal. This 

 deposit is platinum in a state of minute division emanating from the 

 wire, which becomes black and rough as if corroded. The minute 

 particles of platinum are deposited in a lateral direction from the 

 negative wire, and consequently in a different, manner from what is 

 described as occurring in the voltaic arc (De la Rive's ' Electricity,' 

 vol. ii. p. 288), so that the luminous appearance of discharge from 

 the induction-machine can in no way arise from the emanation of 

 particles of the metal. 



The author describes a series of experiments made in the apparatus 

 first prepared, by which the mercury is lowered or raised in the 

 vacuum tube ; he describes the peculiar appearance when the mer- 

 cury is made either positive or negative. In some instances, and 

 particularly when, instead of wires, platinum balls |th of an inch in 

 diameter were used for terminals, the stratifications instantly ceased 

 when the mercury rose above the negative ball; but when the pole 



