(30) 
diameter of the jacket. Hence as soon as the temperature is some 
degrees higher than the temperature of the room, — and such a differ- 
ence is sure to occur in the course of the year, if we want to adjust 
at a definite temperature within the limits of the temperature of 
the room — the cooling will become considerable’). 
Most of the thermostates used allow an accurate regulation only 
at a small loss of heat, moreover on account of the given dimen- 
sions of the bath and because the bottom cannot be heated (for 
instance by a gas flame), only electrical heating could be applied. 
In a narrow high jacket it is very difficult to apply heat by means 
of an electrically heated spiral wire without damaging the illumi- 
nation of the observation tube, for that tube must remain visible 
over a length of 54 c.m. and strong local heating must be avoided as 
it would not be easy to distribute the heat equally by moderate stirring. 
Therefore Rotue’s method ®, followed in the Physikalisch-Technische 
Reichsanstalt, could not be used. Thus there only remains heating 
by means of an alternating current, either the bath itself 3), or separate 
heating tubes filled with a liquid*). But both methods are only 
fit for large baths, as the regulation of the alternating current’) with 
a sufficient degree of accuracy in the case of small baths with a 
rapid loss of heat offers difficulties. 
So we are obliged to continually replace the slightly cooled water 
by new water at the exact temperature and therefore to connect the 
observation bath with a heating bath. Cady *) brings about a circu- 
lation by means of a rapidly rotating funnel, in the bath itself, of 
which means we could not avail ourselves owing to the narrow bore 
of the vessels. For heating microscope stages convection currents 
are used’) which however gives a much too weak and un-reliable 
circulation in this case. 
For the arrangement used by me the adjustments at every tem- 
1) In order to avoid a possible error resulting from the uncertainty in the co-efficient 
of dilatation, all measurements for the isotherm at an ordinary temperature are made 
at the same temperature. 
*) Ein Thermostat mit elektrischer Heizvorrichtung für Temperaturen bis 500°. 
Zeitschr. f. Instr, 1899. 
3) Duane and Lory (Am. Journ. of Se. (4) 9. p. 179, 1900) use a solution of 
common salt of 160 liters and send through it an alternating current of 110 Volts. 
*) Compare the footnote on Qurnt’s method. 
5) Done automatically by Duane and Lory, by Quinr with the hand. 
6) Capy, Journ. of Phys. Chem. 2, pag. 242, 1898. 
7) Van Riyyn, Mech. Zeitung 1899. FrrseNrus’ Zeitschr. 99, p. 96. 
