With the formula below, freezing-point depression can be used to measure the degree of dissociation or the molar mass of the solute. This kind of measurement is called cryoscopy (Greek "cryo" = cold, "scopos" = observe; "observe the cold") and relies on exact measurement of the freezing point. The degree of dissociation is measured by determining the van 't Hoff factor "i" by first determining "m"B and then comparing it to "m"solute. In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing "m"B with the amount of solute dissolved. In this case, "i" must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was, but it was included in textbooks at the turn of the 20th century. As an example, it was still taught as a useful analytic procedure in Cohen's "Practical Organic Chemistry " of 1910, in which the molar mass of naphthalene is determined using a "Beckmann freezing apparatus".