210 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
is shown in the diagram, fig. 62. Fig. 62a and 62b, copied from photo- 
graphs obtained, the former in 1 5 seconds, the latter in 1 0 seconds, show 
respectively the appearances at another phase, when the small tube was 
positive or negative respectively. 
Spottiswoode Tube 1 47, with a Shifting Terminal . 
Carbonic Acid. 
203. — It will be seen on reading the foregoing histories, especially those of tubes 
129, 139, 130, that the strata all take their origin at the positive terminal, 
and that first one, then two, three, and so on, make their appearance as the 
pressure of the gas is diminished. A glance at Plate 15 will render this 
evident, all the strata having their origin at the positive terminal. But 
Mr. Spottiswoode’s elegant contrivance of a moveable terminal shows this to 
be the case in the most conclusive manner ; * we give in Plate 1 7, figs, a, b, c, 
d, e, f, g, h, a representation of the strata obtained, in eight different 
positions of the terminal, in a tube we had made after his model by his 
permission; it is 56 inches long and 1'375 inch diameter, and is shown in 
figs. 39 and 40, page 164. Plate 17 does not give the first phase — a single 
stratum — formed on the positive terminal a certain distance from the negative, 
which distance remains constant, as is shown in the plate. 3240 powder 
cells were used with a resistance of 200,000 ohms to produce a steady 
discharge. 
* Gassiot (Phil. Trans., 1858, p. 12) used a tube attached to a mercurial cistern which was connected 
with an air-pump ; as the pressure was reduced in this cistern, mercury, which originally filled the tube 
supported in a vertical direction, descended, producing a Torricellian vacuum of less or greater length. 
The surface of the mercury acted as the negative terminal. He says : — 
“ It is curious to observe the stratifications retreating from the negative as the mercury ascends the 
tube, or following it as they descend when the vessel is being exhausted, the dark line of discharge 
being compressed or expanded in proportion as the length of the stratification is increased or 
decreased.” 
Also in speaking of the stratified discharge as affected by a moveable glass ball (Brit. Ass. Aberdeen, 
vol. xxix., 1859, sect. p. 11), he says : — 
“ I have already stated that the stratifications near the positive wire are indistinct; but if the glass 
bead is placed near the positive wire and then allowed slowly to descend towards the negative, the 
stratifications at the positive are at first clearly defined near that terminal as at the negative, and as the 
bead rolls gently down, they have the appearance of following the bead and issuing one after the other 
from the positive wire until the bead reaches to within a few inches of the negative, when this action 
gradually ceases. If the tube is now inclined so as to allow the glass bead to return in the contrary 
direction the stratifications appear to recede, becoming more and more clearly defined, until the bead 
passes the positive terminal wire, when the entire discharge returns to its normal state,” 
