U Mr. W. B. Hardy and Lieut, J. K. Hardy 



Glycerine therefore belongs to the class of inactive fluids 

 as defined above. 



By looking for the Newtonian colours one can form some 

 conception of the depth of an inactive fluid which must be 

 maintained between the plates to permit freedom of move- 

 ment. With strong ammonia ("880) most violent seizing 

 -occurred so soon as these colours began to appear. With 

 water, seizing was perhaps less violent and Newtonian colours 

 well seen before it took place. 



Active Fluids are those which facilitate slipping \vhen 



present on the surfaces in layers of any thickness. They 



operate when the layer is of insensible thickness and of the 



order of 1 micron in depth, it may be claimed therefore 



that it is not qua fluid that they act, but because they react 



with the solid face to form a composite surface having a 



lower available energy and therefore a lower capacity for 



seizing. 



The active substances differ amongst themselves, as might 



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be expected, in the extent to which they reduce friction; 

 some only are capable of reducing it to the limit, noticed in 

 a previous section, at which a slow steady glide is possible. 

 Threshold value in grammes per gramme normal pres- 

 sure : — 



Film. Smear. Flooded. 



Acetic acid "62 "7-3 *73 



Butyric acid "5 - 5 — 



Oleic acid 1 "1 "17 



Sulphuric acid '58 'lb 



Strong Hydrochloric acid — — *63 



Trimethylamine, strong solution ... — . — - 6 



Triethylamine *89 '32 



Tripropylamine - 26 — *27 



Pyridine (impure sample) — 46 — 



Castor Oil -1 1 -1 



"Paraffin" -29 15 -22 



The influence of chemical constitution is seen in ths 

 sharply contrasted action of acetic acid and ammonia, the 

 former is active the latter inactive ; in the alkaline series, 

 ammonia, trimethylamine, triethylamine, and tripropylamine. 

 It will be noticed, too, that friction slightly increases as the 

 quantity of an acid present on the plate increases, and 

 -decreases slightly as the quantity of an alkali present 



