Final temperature after caustic dilution: Interpreting the graph I found the below graph in the Occidental Handbook on Caustic. 

I'm confused, what may be the parameter noted on the four curves? 80-100-120-140 F?

The final Temperature is on the y-axis &amp; initial water T is fixed as 70 F.
The labels on the curves are the initial temperatures of the 50% caustic soda solutions.