such vaporization point changing as a function of ambient pressure the phase transition gels in this invention are multicomponent polymers which can be made to respond with various volume changes to a singular external temperature stimuli advantageously the difference in volume between the expanded phase of these phase transitions gels and the contracted phase thereof can be orders of magnitude examples of suitable phase transition gels are disclosed in tanka et al u s pat nos 4 732 930 re 35068 and 5 403 893 and schiller et al wo 96 02276 turning particularly to fig3 it is to be noted that a generally cylindrically shaped fill tube 56 having inlet and outlet ports 58 extends into chamber 43 a fill tube 56 cooperates with elastomeric member 48 to form reservoir 46 when as previously mentioned the beneficial agent is introduced into the fill tube 56 by the novel fill means of the invention with the construction shown in fig3 when the heat expandable mass 52 is heated by the body heat of the ruminant it will expand pand and act upon distendable member 48 in a manner to tend to return the member toward its starting configuration and to controllably force the fluid u201c f u201d which is contained within the reservoir 46 outwardly of the device through the delivery means and into the rumen of the animal it is to be noted that as member 48 moves toward its starting configuration see fig4 and 16 it will closely conform to the shape of the fill tube 56 as the heat expandable mass 52 expands thereby causing a complete and controlled.