"It is a clear oily liquid, having a specific gravity of not less than 0.840 and boiling not below 360° C. (680° F.). It should be free from colored, fluorescing, and odorous compounds.
"When heated for a day by means of a water bath, the paraffine should not become dark colored, and the sulphuric acid should become only slightly brownish. Metallic sodium treated in a similar manner should retain its metallic lustre. Alcohol boiled with paraffine should not have an acid reaction."
72. The Properties of Solid Paraffine (13) are given as follows:[16]
"The melting point of commercial paraffine varies much. Obtained from the residuum of petroleum distillation it is usually 43° C. (109.4 F.), or somewhat higher."
The acid and metallic sodium tests given for liquid paraffine will apply to the solid paraffine.
73. The Value of a Lubricant as a lubricant is independent of the market price; and it is at a maximum, according to Thurston, when it possesses the following characteristics:
1. Enough "body," or combined capillarity and viscosity (82), to keep the surfaces between which it is interposed from coming in contact at maximum pressures.
2. The greatest fluidity consistent with the preceding requirements, i. e., the least fluid friction allowable.
3. The lowest possible coefficient of friction under the conditions in actual use, i. e., the sum of the two components, solid and fluid friction, should be a minimum.