Option Explicit

' Linear Interpolation function from the companion CD 
' of the book "Excel for Scientists" by Billo.
Function InterpolateL(lookup_value, known_xs, known_ys)

  Dim pointer As Integer
  Dim X0 As Double
  Dim Y0 As Double
  Dim X1 As Double
  Dim Y1 As Double

  'Following line added to prevent extrapolation
  If lookup_value < Application.Min(known_xs) Or lookup_value > Application.Max(known_xs) Then
    InterpolateL = CVErr(xlErrRef): Exit Function
  End If
  
  pointer = Application.Match(lookup_value, known_xs, 1)
  X0 = known_xs(pointer)
  Y0 = known_ys(pointer)
  X1 = known_xs(pointer + 1)
  Y1 = known_ys(pointer + 1)
  InterpolateL = Y0 + (lookup_value - X0) * (Y1 - Y0) / (X1 - X0)
End Function

ethylene_glycol_section.txt

Freezing and Boiling Points of Heat Transfer Fluid;
Wt% Ethylene Glycol;FreezingPoint °F
0.0;32.0
5.0;29.4
10.0;26.2
15.0;22.2
20.0;17.9
21.0;16.8
22.0;15.9
23.0;14.9
24.0;13.7
25.0;12.7
26.0;11.4
27.0;10.4
28.0;9.2
29.0;8.0
30.0;6.7
35.0;-0.2
40.0;-8.1
45.0;-17.5
50.0;-28.9
55.0;-42.0
60.0;-54.9