It has been shown that a diamond sliding on glass can under suitable conditions induce surface flow of the glass. The speed (v) and load (W) necessary to cause this flow have been studied and it is found that flow only occurs if v $\surd $W exceeds a critical value which depends on the type of glass. There is evidence that flow occurs when a critical surface temperature is reached and approximate calculations indicate that this temperature corresponds to the softening point of the glass. It is suggested that this type of thermal softening and flow may occur on a submicroscopic scale when glass is polished. Heavy wear of a diamond sliding on glass has been observed. This wear is very dependent on the humidity of the atmosphere; in a dry atmosphere it may be 100 times greater than it is in a humid one. The wear rate on glass in dry air is greater than that on a scaife (diamond impregnated grinding wheel). The wear is accompanied by the production of a detritus of amorphous carbon so that the process appears to be primarily one of degradation of diamond to amorphous carbon. This degradation is aided by the temperature rise at the rubbing interface and there is evidence that it can occur at temperatures lower than those required when the diamond is heated without rubbing.