LUBRICATING AND OTIIKR PROPERTIES OF THIN OILY KILMS
My first experiments were very simple ones, with a slab of thick plate glass and a small glass bott.lt> weighing about 4 ox. The diameter of the bottle is 4jr cm., and the bottom is concave, bounded by a rim which is not ground but makes a fairly good fit with the plate. The slab is placed upon a slope, and the subject of observation is the slipping of the bottle upon it. If we begin with surfaces washed and well rubbed with an ordinary cloth, or gone over with a recently wiped hand, we lind that at a suitable inclination the conditions are uniform, the bottle starting slowly and moving freely from every position. If now we breathe upon the slab, maintained in a fixed position, or upon the bottle, or upon both, we find that the bottle sticks and requires very sensible, forces to make it move down. A like result ensues when the. contacts are thoroughly wetted with water instead of being merely dumped, When, after damping with the breath, evaporation removes the moisture, almost complete recovery of the original slipperiness recurs.
In the slippery condition the Kurfaces, though apparently clean, are undoubtedly coated with an invisible greasy layer. If, after a thorough washing and rubbing under the tup, the, surfaces are dried by evaporation after shaking oil'as much of the, water as possible, they arc, found to be sticky ,as compared with the condition after wiping. A better experiment was made with substitution of a strip of thinner glass about •"» em. wide for the thick slab. This was heated strongly by an alcohol flame, preferably with use of a blow-pipe. At a certain angle of inclination the bottle was held everywhere, but. on going over the surface with the lingers, not purposely greased, free movement ensued. As might have been expected, the clean surface is sticky UH compared with one slightly greased ; the difficulty HO lur in to explain the effect of moisture upon a surface already slightly greased. It WUN not wur-that- the effect, of alcohol was tumihir to that of water.
At thia stage it waH important to make mire that the Hfcickirieaa duo t(» water was not connected with the minuteness of the quantity in operation. Accordingly a ghiHH plate was mounted at a mutable angle in a dish filled with water. Upon this fully drowned surface the bottle stuck, tin* inclination biting Buch that on the slightest greasing the motion became free. In another experiment the water in the dish was replaced by paraffin oil. There, was decided stickiness as compared with surfaces slightly greased.
The better to guard against the ordinary operation of mirf'aee tension, tho weight of the bottle was increased by inclusion of mercury until it reached 20 08., but without material modification of the effects observed. The moisture of the breath, or drowning in water whether clean or soapy, developed the name stickiness aa before.
The next series of experiments was a little more elaborate. In order to obtain measures more readily, and to facilitate drowning of the contacts, the
IIs than 47^/c. W. P. S.]ng there is some light inxxix. p. 128, 1874). I do not know the date of Thoulet's use of the solution, but suspect that it was subsequent to Sonstadt's.