In　order　to　obtain　the　fundamental　data　to　discuss　the　mechanism　of　vapor　explosio血　caused　by the　abrupt　destruction　of　phase　equilibrium，　a　rupture　disk　was　bfoken　undervarious　conditions　of sevefal　kinds　of　vessels　in　which　the　saturate　water　at　high　pressure　was　contained　and　pressure change　was　measured、　The　results　were　summerized　as　follows： （i）The　phenomena　caused　in　the　vessels　were　explained　from　the　pressure　measurements． （ii）The　evaporation　ratio　alld　the　volume　ratio　of　water　were　calcurated　from　the　initial　conditions，The　maximum　pressure　obtained　in　the　experiment　was　related　to　the　volume ratio.（iii）The　relation　between　opening　hole　ratio　and　maximum pressure　and　weight　of　water　left over　were　discussed．The　pressure　drops　were　related　to　peak　overpressures， （iv）The　limits　above　which　the　overpressures　occured，　were　obtained　in　the　velocitles　of　pressure　drops　and　the　scaled　pressure　drops.

As described previou~ papers^1）-5）, the stability index μ for a chemieally reactive system is impor tant to evaluate its stability. The stability index had been.shown to be obtainable fram the temperature change of the systenl against that af the environrnent at steady states¹⁾. The derivation of μ by this method was shown and made it rather difficult to trace the change in the stability of a system. In the present paper, transient method was proposed to derive μ rapidly. ( i ) When a step of temperature change in the enviranment was given, the temperature transient of the system provided μ and time constant by a simple canstruction proposed by the authors as given in Figures 1 and 2. ( ii ) When the environmental temperature changed sinusoidally, both amplitude of the system response ~nd phase lag gave time constant and μ. (iii)When the environmental ternperature fluctuated arbitrarily, the modefied phase space plot provided μ and r easily.

Being　inherited　from　the　previous　paper，a　thermally　Grst　order　system　was　analyzed　theoretically with　the　emphasis　on　safety　engineering。　The　thermal　responses　to　a　step　excitation　were　brought into　consideration．　The　mathematical　relations　were　modified　into　convenient　forms　to　evaluate　the danger　oheactive　systems．　In　paticular，a　few　diagramatical　analyses　of　practical　data　were　proposed to　derive　the　safety　indexes.　The　mathenlatical　equations　and　variables　were　projected　on　the　practical models. The predlctions of potential hazards on the mathematical analyses were shown to be feasible．