IF F.A RSON \y I JVC VBA TOR. 
65 
Just as water boils at 212'-’, so sulphuric ether boils and expands into vapour at 94^. Other 
liquids boil at higher temperatures, and as a mixture generally boils at a heat inter- 
mediate between that of its two components, it is easy to prepare a slightly modified 
ether which shall boil (at ordinary barometrical pressures) at 98^ or 99*“’, the lowest 
admissible incubator temperature. Mr. Hearson’s regulator consists of a few drops of such 
volatile fluid enclosed between two flat brass plates, soldered together all round their edges into 
a closed flat capsule. Then, directly a heat of 98° is exceeded, the two plates “bulge” under the 
ether vapour which is formed ; and hence we have a very powerful force, which acts instantly on a 
given temperature being attained. The incubator is shown in section in Fig. 40. A A is the tank 
of water, much smaller than in preceding machines, traversed by the flue I. W from the lamp T. 
The flue really returns through the tank, so that the outlet W is on the same side as T ; but this 
Fig. 40. 
cannot be shown with clearness. B is the concave egg-tray of perforated zinc, supported in a 
drawer floored with open strips of wood, K. The concavity brings the outer eggs rather nearer the 
heat, and obviates the necessity for moving the eggs about except in turning them. Air enters, as 
in the preceding machines, through the hole D in the bottom ol the incubator, having to pass 
through canvas soaking in the water-troughs, C C, whence it passes, impregnated with moisture, to 
the drawer, escaping by the ventilating holes E E. The whole is surrounded as usual by packing. 
N is a thermometer. 
The regulation is exquisitely simple and easily understood. The lamp T has a vertical flue, V, 
above it, as well as the heating-flue L ; and if this be opened, of course nearly all the heat escapes 
by preference vertically, instead of passing through the tank. This flue V is closed by a flap-valve, 
F, at the end of a lever, G. Near the pivot end of the lever at P is attached a stiff lifting-wire, 
which passes through a tube, O, in the centre of the tank ; and the bottom of this wire rests on the 
capsule, which is simply laid on a small rigid tabic at S. As the capsule bulges, therefore, it lifts 
p and F. If the machine were started thus, the heat would therefore rise to 98°, and at this point 
the valve F would open. But the sliding weight II allows more pressure to be put upon the 
capsule, which has the effect of raising the boiling-point (the boiling-point of water rises about if 0 
for every inch pressure of the barometer). In this way, therefore, the boiling point may be set 
