3 0 to 5 5 change in elongation and 0 to 1 6 change in volume after immersion in various solvents such as 70 iso octane with 30 toluene for 70 hours at room temperature such material is commercially available from parker seal company of culver city california under the tradename viton and the product numbers v747 75 and 77 545 the compression spring means 26 is made of steel with an inside diameter of approximately 0 265 inch so as to permit several turns to be loosely slidably received circumjacent the cylindrical spring receiving stub shaft portion 74 of approximately 0 205 inch diameter the inner end of the spring is seated on washer means 28 and the effective length of the spring is such as to be compressed and to exert a closing force on the valve plunger means 22 in the closed position of fig2 thus the spring exerts a continuous closing force on the valve plunger means and is of suitable strength to obtain the desired sealing effect in the closed position while permitting relatively easy opening by cattle rubbing against the rounded end surface 80 the outside diameter of the spring is substantially less than the inside diameter of the chamber 36 whereby sufficient relative movement is permitted to prevent jamming as the cattle push roll and bang against the apparatus the retaining washer means 28 is also made of steel and is held against end wall 38 by the compression spring an inlet opening 90 in the washer permits the chamber 36 to be filled with oil and insecticide through inlet opening 42 from a reservoir not shown which fills the space 92 surrounding the valve assembly by gravity feed in a conventional manner while the inventive.