These complete thermostat units have been used for over 60 years for regulating temperature.
The wafer contains ether inside which causes the wafer to expand when heated and contract when cooled.
The wafer sits in a bracket opposite a micro-switch which is pressed in by the wafer as it heats to a certain point and turns off the power flowing through it and is released when the wafer contracts allowing power to flow again.
By wiring your heaters to this, the temperature can be set by screwing the wafer closer to or further away from the switch so it has to expand more or less before turning the power off.
These units do have a higher degree of variance of temperature due to the mechanical nature of their operation than an electronic thermostat but there is also less to go wrong with them and they are less subject to damage from power surges and similar electrical issues.
These thermostat will allow setting a temperature roughly between 20 and 40 C depending on the system they are placed into. They are often used to replace old thermostats in incubators and can also be used to build your own incubator, brooder or intensive care unit.