detecting instrument a conventional coil will be disposed in spaced relation around the sensing element it will be understood that this sensing element can be used in a halogen vapor detector such as that shown in the patent referenced immediately above or a mixture of sensing material prepared according to the invention may be formed in alternative desired configuration for example there is shown in relatively recently issued u s pat no 3 991 360 a generally cylindrically shaped halogen sensing element that is formed by such a method and further characterized by fusing the treated granular alumina into a tubular element and tightly wrapping it with a heater coil to construct the invention disclosed and claimed therein now bearing in mind the types of sensing devices that may be made from granular mixtures of sensing material manufactured by the method of the invention reference is made to fig1 of the drawing which lists a preferred series of method steps for practicing the invention thus it will be seen that according to the method of the invention one first provides a batch of granular activated alumina al 2 o 3 we have found that commercially available activated alumina granules sold under the tradename alcoa type f 1 is suitable for use in the method of the invention the granules of this type of activated alumina are sufficiently porous to provide an unusually large surface area on which alkali metal is subsequently deposited also the granules of alumina are rugged enough to prevent their being broken down.