7. A high temperature of vaporization and a low temperature (83) of solidification.
8. Special adaptation as to speed and pressure of rubbing surfaces under which the unguent is to be used.
9. It must be free from grit and from all foreign matter.
The author will add that for use in horology:
10. It must possess a minimum variation of viscosity (84) in varying temperatures.
The writer can see no reason why a mineral oil which has been properly refined _and of the proper consistency_, either alone or mixed with animal oil, could not be used to great advantage in horology. Indeed, the possibilities in this direction seem to be so pregnant with promises of good results that some space will be devoted to the matter.
~74. The Special Advantages of Mineral Oils~ as lubricants in horology are:
1. Mineral oils can be made entirely pure, and possess uniform and known properties when derived from the same or a similar source; while the quality of animal and vegetable oils varies from year to year, depending, in animal oils, on the season of the year when the crude oil is obtained, on the age and condition of the animal, and on the kind, quality and quantity of food which it had (5) recently consumed; and in vegetable oils on the season, soil, climate and method of treatment.
2. According to Thurston "All vegetable and animal oils are compounds of glycerine with fatty acids. When they become old, decomposition takes place and the acid is set free, by which action the oils become rancid. This rancid oil or acid will attack and injure machinery. Again, all animal oils contain more or less gummy matter, which accumulates when exposed to the action of the atmosphere, and will, consequently, retard the motion of the machinery."
3. Spon, in his Encyclopedia of the Arts, gives his views to the effect that "The best oil is that which has the greatest adhesion to metallic surfaces and the least cohesion in its own particles. In this respect fine mineral oils stand first, sperm oil second, neatsfoot oil third. Consequently the best mineral oils are the best for light bearings. The best oil to give body to fine mineral oils is sperm oil."
4. "Mineral oils do not absorb oxygen," and consequently do not "gum" or become viscous.--Thurston.