In testing lubricants in general, a great deal depends upon the class of work in which they are to be employed. In dealing with lubricating greases the specific gravity should always be determined. The viscosity is, of course, also a matter of the utmost importance. If possible the viscosity should be taken at the temperature at which the grease is to be subjected when used, but this cannot always be done; 300° F. will be found to be a very suitable temperature for the determination of the viscosity of heavy lubricants. Although one of the standard viscosimeters is the most satisfactory instrument with which to carry out the test, yet it is not a necessity. Provided the test be always conducted in exactly the same manner, and at a fixed temperature, using a standard sample for comparison, the form of apparatus used is not of great importance. Most dealers in scientific apparatus will provide a simple and cheap instrument, the results obtained with which will be found reliable. With the exercise of a little ingenuity any one can fit up a viscosimeter for himself at a very small outlay.

Acidity is another important point to note in dealing with lubricating greases. Calculated as sulphuric acid, the free acid should not exceed .01 per cent, and free fatty acids should not be present to any extent. Cylinder oil should dissolve completely in petroleum benzine (specific gravity, .700), giving a clear solution. In dealing with machine oils the conditions are somewhat different. Fatty oils in mixture with mineral oils are very useful, as they give better lubrication and driving power, especially for heavy axles, for which these mixtures should always be used. The specific gravity should be from .900 to .915 and the freezing point should not be above 58° F. The flash point of heavy machine oils is not a matter of great importance. The viscosity of dynamo oils, taken in Engler's apparatus, should be 15-16 at 68°F. and 3.5-4 at 122° F. In dealing with wagon oils and greases it should be remembered that the best kinds are those which are free from rosin and rosin products, and their flash point should be above 212° F.

To Test Grease

To be assured of the purity of grease, its density is examined as compared with water; a piece of fat of the size of a pea is placed in a glass of water. If it remains on the surface or sinks very slowly the fat is pure; if it sinks rapidly to the bottom the fat is mixed with heavy matters and coom is the result.