220 Wetting Poirer and its iJitcniii nation 



water, may be quite ineffective as a dip, whilst a solution of the same 

 concentration, but containing in addition a small percentage of soap 

 and oil to increase the wetting power, proves to be quite satisfactory. 

 The fact that the Queensland official dip contains soap as a basis may 

 be quoted as an indication tliat the inijiortancc of the wetting power of 

 dipping fluids is recognised in actual practice. 



The difficulty of determining the wetting ])ower of a solution has, 

 however, precluded this point being considered in the comparison of 

 dips in the laboratory. 



What has been said of dipping fluids, refers with equal force to 

 horticultural spraying fluids — a high percentage of loxic substance is no 

 criterion of the efficiency. The supreme importance of the spray fluid 

 having a high weiting poiver is gradual!)' becoming more and more 

 recognised ; though not generally by Government authorities. In the 

 United States, compulsory spraying is rapidly becoming the rule, and 

 the standardisation of spraying fluids is its natural consequence. This 

 standardisation merely considers the percentage of the toxic agent, and 

 entirely disregards the equally imjjortant point of the wetting power. 



A simple laboratory test for the determination of the relative wetting 

 powers of dift'erent preparations is in urgent demand, and various 

 methods of comparison have been suggested from time to time. 

 Generally speaking, these suggestions have not been based upon an 

 exact knowledge of the principles underlying the process of wetting, 

 and, in consequence, the results obtained by the use of such methods 

 are of little value. 



The generally accepted test is the determination of the surface 

 tension of the preparation. Thus, for example, Briinnich and Smith 

 (1914, p. 83) state that 'the wetting power of any liquid, or its property 

 to form a uniform film upon a greasy surface, depends primarily upon 

 its surface tension,' and they attempt to compare the wetting power of 

 various fiquids by measuring their surface tension by means of a 

 stalagmometer. Vermorel and Dantony (1910, pp. 1144-5) distinguish 

 between the static and dynamic surface tensions of soap solutions, and 

 claim that the value of the static tension, as opposed to the true (or 

 dynamic), affords a satisfactory criterion of the wetting power. They 

 point out that two solutions of sodium oleate of concentration 5 % and 

 0-1 % respectively, give the same capillary rise and w-et equally well ; 

 nevertheless their true surface tensions differ very considerably. In a 

 later paper, however (1912, pp. 1300-1), they state that 'the surface 

 tension of a fiquid is not sufficient to determine its wetting power.. . . 



