
PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 725 
dusk, there are very sudden changes, partly due to increased pressure in the 
gasometers, partly to the rapid lighting of 
many burners. The process employed by Crum 
Brown is to cut off, or increase, the supply of 
gas to a small gasholder by a sort of valve 
which acts almost instantaneously. The valve 
consists of an india-rubber tube, which is just 
on the point of being nzpped—that is, being 
bent over so as almost completely to close it. 
A very slight motion of one end effects the 
difference between nipping and comparative 
openness, so that when this tube is appended 
to one of the weights of the gasholder, it main- 
tains a perfectly regular pressure in the holder. 
In fact, it was not possible to observe, from 
half-hour to half-hour, any variation of level 
of the inverted vessel. 
The theory of this application is that, where absolute regularity or steadiness 
cannot be had, the best substitute for it is extreme stability of equilibrium. 
There is, no doubt, a constant change going on, but any displacement produces 
such a disproportionately great force of restitution as practically to keep every- 
thing steady. | 
§ 9. Another source of great difficulty, which had been fully felt by Forsss, 
was the heating of the short bar. The method he finally adopted is perhaps 
not applicable, except to iron: at least when high temperatures are required. 
He plunged his iron bar bodily into a bath of melted fusible metal. The bar 
was wrapped in paper to prevent too sudden an abstraction of heat from the 
melted metal. I first tried to heat the bars by means of a sort of air-bath, but 
I found that in such a bath they all became oxidised before the temperature 
_ was sufficiently raised. I endeavoured to overcome this difficulty by putting 
successive covers on the bath, making it, in fact, almost air-tight, and passing 
a uniform current of dry carbonic acid gas through it. | 
These methods proved comparative failures, and the simple process ulti- 
mately adopted consisted in taking a brass gas-pipe, pierced along its upper side 
by a number of holes at equal intervals from one another. This burner was 
connected directly with the gasometer and produced a row of little jets. As 
these were of gradually diminishing intensity (in consequence of diminishing 
pressure), the tube was slightly inclined upwards from the gasholder. The bar 
(previously raised to a temperature of about 100°, by radiation from a 
fire, to prevent deposition of moisture from the flames) was placed over it 
in a horizontal position on a sort of rack, on which it was kept turning 
VOL. XXVIII. PART III. 9D 





