FUNCTION HIDALG(IB,FR)
C     ======================
C
C     Read table of wavelengths and photo-ionization cross-sections
C     from Hidalgo (1968, Ap. J., 153, 981) for the species indicated by IB
C     (Hidalgo's number = INDEX = -IB-100).
C     Compute linearly interpolated value of the cross-section
C     at the frequency FR.
C
      INCLUDE 'PARAMS.FOR'
      DIMENSION WL1(20),WL2(20),WLI(20),SIG0(20,24),SIGS(20)
C
      DATA WL1 /
     *  39.1, 80.9, 97.6,100.1,104.3,107.2,108.7,111.9,113.6,115.4,
     * 117.1,119.0,124.8,126.9,129.1,131.3,133.6,136.0,138.5,141.1/
      DATA WL2 /
     *  68.5, 80.9,100.1,120.9,158.8,165.7,177.3,190.6,200.7,206.2,
     * 211.9,218.0,224.5,231.3,246.3,5*0./
      DATA SIG0 /
     *120*0.,
     *.0460,.2400,.3500,.3700,.4000,.4300,.4400,.4600,.4700,.4900,
     *.5000,.5200,.5700,.6200, 6*0.,
     * 80*0.,
     *.0092,.1000,.1900,.2100,.2300,.2500,.2600,.2900,.3000,.3200,
     *.3400,.3500,.4100,.4300,.4500,.4800,.5000,.5300,.5600,.5900,
     * 20*0.,
     *.3400,.4600,.6300,.7700,.9100,1.080, 14*0.,
     * 20*0.,
     *.0064,.1100,.2200,.4100,.9400,1.000,1.300,1.600, 12*0.,
     * 80*0.,
     *.0370,.0650,.1300,.2400,.5500,.6300,.7700,.9500,1.100,1.250,
     * 10*0.,
     * 40*0.,
     *.0220,.0390,.0800,.1500,.3500,.4000,.4900,.6200,.7200,.7800,
     *.8500,.9300,1.020,
     * 7*0./
C
      INDEX=-IB-100
      NUM=20
      IF(INDEX.GE.13.AND.INDEX.LE.27) NUM=15
      DO 10 I=1,NUM
         IF(INDEX.LT.13) WLI(I)=WL1(I)
         IF(INDEX.GE.13) WLI(I)=WL2(I)
         SIGS(I)=SIG0(I,INDEX)
   10 CONTINUE
C
      WLAM=2.997925E18/FR
      IL=1
      IR=NUM
      DO 50 I=1,NUM-1
        IF(WLAM.GE.WLI(I).AND.WLAM.LE.WLI(I+1)) THEN
          IL=I
          IR=I+1
          GO TO 60
        ENDIF
 50   CONTINUE
C
C     LINEAR INTERPOLATION:
C
 60   SIGM=(SIGS(IR)-SIGS(IL))*(WLAM-WLI(IL))/(WLI(IR)-WLI(IL))
     *      + SIGS(IL)
C
C     IF OUTSIDE WAVELENGTH RANGE SET TO FIRST(LAST) VALUE:
C
       IF(WLAM.LE.WLI(1)) SIGM=SIGS(1)
       IF(WLAM.GE.WLI(NUM)) SIGM=SIGS(NUM)
C
C     IF LAST NON-ZERO SIG VALUES, NO INTERPOLATION:
C
c       IF(SIGS(IR).EQ.0.) SIGM=SIGS(IL)
C
      HIDALG=SIGM*1.E-18
      RETURN
      END