202 Mr. W. B. Hardy on 



may be handled freely if actual contact with the burnished 

 face is avoided. To clean the faces the plate and slider 

 were washed in ethyl alcohol 98 per cent., drained, and 

 burnished with wash-leather. Wash-leather clings to a 

 clean face of bismuth. When the surface is rubbed with it, 

 there is a characteristic harsh feel and characteristic notes 

 of high pitch are given out if the surface be clean. Rarely 

 a lubricant is found which will not be displaced from the 

 surfaces in this simple way : it is then necessary to clean 

 them with rouge. 



What is the test of cleanliness ? There is no criterion 

 other tban static friction itself. When static friction is 

 maximal it is assumed that the surfaces are clean, or, to 

 put it more exactly, " clean " faces are defined as those 

 whose static friction reaches a certain level such that a 

 force of 34,300 dynes just fails to produce movement in 

 the slider used throughout, whose weight was 70*5 grammes 

 and radius of curvature 25'5 millimetres. This assumption 

 is justified by the fact that every substance tried was found 

 to reduce this datum value when applied to the surface. 

 In this bismuth differs from glass. Many substances are 

 neutral to glass in that they do not alter the static friction. 

 No substance neutral to bismuth was found. 



This section, with the limitations noticed in paragraph 2, 

 is confined to a study of one variable — namely, the chemical 

 constitution of the lubricant. One point should, however, 

 be mentioned and reserved for future discussion. A few 

 lubricants appear to abolish the static friction of bismuth 

 altogether (e. g. ricinolic acid). In these cases the value 

 given is that at which the tractive force produced sliding 

 so slow as to be just detectable by unaided vision. 



In testing a fluid the surfaces were flooded so that 

 the slider moved in a pool. The thickness of the film of 

 lubricant then was that determined naturally by capillarity 

 acting in opposition to the normal pressure. Solid lubricants 

 were deposited in a layer, thick enough to dull the burnished 

 surfaces, from dilute solutions in ethyl alcohol, benzene, or 

 ether. There is a danger lest the friction of bismuth against 

 the solid in mass be mistaken for what is sought for, namely, 

 the friction of bismuth against bismuth lubricated by the 

 solid. The former obviously is an important limiting value 

 when the lubricant is a solid. Whether it is the only value 

 and identical with the latter will be considered on some 

 future occasion. In the meantime it may be noted here 

 that the value of the friction of bismuth lubricated with 



