The strength of a soap-making lye is often given in degrees Tw.; this refers to the density of the lye as indicated by a Twaddel hydrometer, the instrument generally used for the purpose in soap-making. The hydrometer is immersed in the lye, whose density can then be read off from the scale at the top of the instrument. Twaddel hydrometer degrees are converted into specific gravities by multiplying by '005 and adding 1. Thus the specific gravity of, say, 65° Tw., is: (65 x .005) + 1 = 1325.