“Wouldn’t it be a bit hard for us to heat anything up to a thousand degrees?” asked Herb.
“Not a bit of it!” exclaimed Doctor Dale. “Why, the coal fire in a kitchen range will heat up to that temperature easily, or we could reach it over a good gas flame. The main difficulty is, not to heat the galena sufficiently, but to cool it off properly. If it cools too fast, the galena in the crucible will be made up of very small grains with only a few sensitive surfaces.”
“How could you tell when you had reached the proper heat?” asked Jimmy. “I should think it would take a pretty good thermometer to register that high.”
“It would take a better one than I have ever seen,” replied Doctor Dale, with a smile. “As it happens, however, it would not be necessary to depend on any heat-measuring device. The color of the crucible would be an accurate enough guide for our purposes.”
“How is that?” put in Herb, deeply interested.
“When it showed a dull red,” was the answer, “you could safely estimate a heat of about seven hundred degrees, and at one thousand degrees it will be somewhere between a bright red and a deep orange color. When it gets to this point it should be kept there for about an hour, after which you can begin a very slow cooling process.”
“Wouldn’t that be hard to do?” asked Joe.
“The best way, if your crucible is in a coal fire, is simply to let the fire go out; but if you are using a gas flame, you can turn it down very gradually and get the desired result just the same,” was the reply.
“Let’s try it tonight!” exclaimed Herb, with his usual impulsiveness.
“I’m afraid we are not equipped for the experiment this evening,” returned the doctor. “If you would really like to try it, however, I’ll have the necessary apparatus here the next time you boys come to see me.”