I have an old "The Free no. 5" treadle sewing machine, and wanted to know if I can use it for leather. What kind of needle should I use, if I can.

Sewing leather in a home sewing machine would need electric to help get through the leather. I had an old Brother portable that I would sew leather on all the time. I would just use the biggest, sharpest needle that I could buy that would fit into the machine. But seeing as yours is a treadle machine you may not have enough umph to do this. Worth a try though. Just try it on a scrap to see if it will go through it. Don't use heavy gauge thread either and don't make stitches too close either. I made a leather chamois shirt for a friend of mine that turned out great and also made myself leather pants, skirt. Also sewed fur very carefully making a bedspread. The pelts were not easy to work with but I was careful not to make stitches too tight or the pelt would split open at the seams.