W. F. Cooper and W. H. Nuttall 237 



used in practice, and 250 c.c. of tlie diluted liquid are placed in a gas 

 jar (20 cm. high x 5-5 cm. internal diameter). The pipette is then 

 filled to above the upper mark {F) with oil, and, after carefully wiping 

 the outside of the pipette and the surface of the orifice, the pipette is 

 lowered into the gas jar and allowed to rest on its bottom. The pipette 

 is maintained in a vertical position by means of a clamp. The top 

 stopcock is then opened and the level of the oil allowed to sink to the 

 upper mark. Any oil adhering to the orifice is carefully removed by 

 means of a glass rod, after which the stopcock is again opened and 

 the number of drops formed whilst the level of the oil in the pipette 

 sinks to the lower mark {G) are counted. 



It may happen, with liquids of very low interfacial tension, that 

 the oil flows from the orifice in a continuous stream instead of forming 

 drops. This occurs when the interfacial tension is so low that the 

 diameter of the drops is less than that of the orifice of the pipette, 

 and with our pipette, it signified an interfacial tension of less than 

 0-66 dynes/cm. If streaming shoidd occur, a pipette with a smaller 

 aperture may be employed or the aqueous liqmd must be further 

 diluted. 



It is advisable to keep a good stock of standard oil so that all deter- 

 minations may be made on the same sample of oil and therefore be 

 strictly comparable. The same lot of oil should not be used for more 

 than one determination. As different samples of liquid vaseline differ 

 shghtly and give somewhat dift'erent drop numbers with distilled water, 

 it is advisable to take the latter as a standard liipiid, and to express the 

 wetting power of the solution as the ratio of the drop number of the 

 solution to that of distilled water, multiplied by 100. The following 

 table contains the results of determination of the wetting power of 

 different cattle dips. The dips were all diluted to the same degree, and 

 the drop number determined at a temperature of 20". 



We do not claim that this means of estimating wetting power is 

 free from every objection, nor can it be regarded as affording absolutely 



