Also when the critical range is reached, a peculiar contraction of size interrupts the gradual expansion which had been developing as the test piece absorbed heat units, and therefore these several observations give us reason to believe that our conclusions as noted above must be more or less correct. Now if all steels acted exactly like the little test piece which we have been observing above as they were placed in the hardening furnace, it would not take us very much longer to finish our preliminary investigations. You remember the piece of steel which we have been investigating was a piece of simple carbon tool steel, containing about 0.90% carbon. But all steels do not contain just this same percentage of carbon, and may also contain various elements other than carbon, all of which produce many and varied results during the process of heating, treating and hardening. In order to better visualize the investigation which we are making, let us picture graphically each step which we take. If therefore, we let the vertical lines represent the different carbon contents which steel might have, and the horizontal lines the different degrees of temperatures through which we might desire to heat the steel under discussion and then plotted the phenomenon described above we would have a picture something as follows: [Illustration: Graph showing the point of recalescence] Now all that picture means is that as we heated up a piece of simple carbon tool steel containing O.9% C, we discovered a certain very noticeable reaction which occurred just about half way between 1250 degrees and 1350 degrees Fahrenheit, which we decided to call the point of recalescence, and then on further heating of the piece no other such phenomenon was noticed. Now let us go through the same experiment with a piece of steel containing .45% C. Yes, just as before, as the temperature 1250 degrees Fahrenheit is reached we note all the strange symptoms which are characteristic of the point of recalescence and then, just as we are about to decide that it is hardly necessary to go further we notice that the pyrometer needle has again come to rest, but that this time it is registering 1390 degrees Fahrenheit. Therefore, it would seem as if this piece had two critical ranges instead of one and we are now quite ready to again proceed with our heating to see if anything else occurs. However, as nothing does happen we turn to our picture and plot the two points just observed, together with the one point found on our first investigation, and the drawing then looks something as follows: [Illustration: Graph additional shows the recalescence of a second sample containing a different rate of carbon]