236 Wetting Power and Its Determination 



aperture, the difference in density and the interfaeial tension. Where 

 it is desired to state the interfaeial tension in absolute terms, all these 

 four factors must be taken into account. The calculations involved 

 are somewhat tedious; they have been dealt witli fully by Lewis (1908), 

 to whose paper those interested are referred. 'Die values given in 

 Table II for the interfaeial tension were obtained by this method. 



Where comparative values only are required, as is the case in the 

 determination of wetting powder, the interfaeial tension may be taken 

 as inversely proportional to the number of drops, since the diameter 

 of the aperture remains constant, and the difference in density between 

 the oil and the aqueous liquids does not usually vary greatly, owing to 

 the high dilution at which the dip or spray fluid is employed. As we 

 have already seen that the wetting power varies indirectly as the inter- 

 faeial tension, it follows that the wetting power is directly proportional 

 to the drop number. 



The form of pipette used for the determination of the interfaeial 

 tension is shown in Fig. 3, which requires little explanation. The 

 lower limb {A) is curved round, and its aperture is somewhat restricted 

 and ground carefully to a plane surface. The walls of the aperture 

 sliould be fairly thin, in order to prevent unevenness in wetting. Both 

 the upper limb of the pipette, as well as the lateral tube with which it 

 is provided, arc closed with glass stop cocks {li and C). The lateral 

 tube is used for filling the pipette by suction. The upper limb ter- 

 minates in a very fine capillary tube {D), which is best attached by 

 means of rubber tubing. The object of this capillary is to regulate the 

 flow of oil through the aqueous liquid, and should be so selected as to 

 allow an interval of about ten seconds between successive drops, when 

 water is used. 



The volume of the pipette used in our experiments was 49-1 c.c, 

 the length from the bottom of the bulb (E) to that of the bend 22-5 c.c. ; 

 the diameter of the aperture was 3-.5 mm. Later experience has indi- 

 cated that a pipette with a capacity of 25 c.c. and an aperture 2-7 mm. 

 in diameter would probably be found most convenient for general 

 purposes. The paraffin oil used as a standard was a liquid vaseline, 

 which was almost acid-free and possessed a density of 0-8690^. 



An actual determination is carried out as follows. The preparation 

 {e.g. dipping fluid) is diluted- to the strength at which it is generally 



' Obtained from tlic Cliosebrough Maiiufaeturing Co., llolljoni Viaduct. 

 • In order to obtain comparable results, the dipping fluid should always be diluted 

 immediately before the count is made. 



