DEVELOPMENT OF A DETERGENT TEST 415 tests may be made for each group of plates available, and that complications due to a large decrease in temperature are lessened. An investigation into the differences between the results of tests using different soils gave a picture far too complicated to be described in the present article. The effect of varying the quantity of soil per plate differed with different soils, as mentioned in the previous section, and the effect of varying concentration of detergent differed with different detergents. The data to be published will therefore be confined to some typical series of results. CONCENTRATION OF DETERGENT All tests reported in the previous articles have been made with a range of four concentrations of detergent. This was done simply to broaden the validity of any conclusions that might be drawn from the data, and the respective performances of the different solutions were not compared. However, when the results are plotted, as in the graphs (p. 416), each curve appears to consist of two linear portions, the first section being steep and passing through, or near to, the origin, whilst beyond the intersection the number of plates increases more slowly than the concentration. The slopes of the first sections of the curves are quite different for the three cases, and the magnitudes of the changes of slope are different. The change in slope of each curve can be identified with the critical micelle concentration which was determined as 1.7 g.p.1. for Nansa L Liquid and 1.2 g.p.1. for the sulphated alcohol,-using the fluorescent dye method. As the sulphated alcohol was a solution of 20 per cent active matter, the change in slope occurs at 0.024 per cent, which agrees with Pugh's statement (see reference 4) of "a linear relationship between the number of plates washed and the concentration over the range 0.025 per cent to 0.1 per cent active matter." To this we might add that the linear relationship must not be confused with direct proportionality, for doubling the concentration from 1-5 to 3 g.p.1. increases the plates from 14 to 20 only. The most significant aspects of the concentration effect as regards con- siderations of a practical test is that comparisons of different detergents made below the critical concentration will bear no resemblance to those made above. This seems not to be generally appreciated, and most of the published data compare different detergents at single arbitrary concentrations which give results so limited as to be misleading. Lloyd E. Weeks, et al., a specify 0-1 per cent active concentration, which is well above the critical concentration and thus is not subject to the same criticisms as the recommended procedure reported by Louis E. Wells.5 The latter uses 0-1 per cent "as is" detergent concentration which will lie above, below, or within the critical region for different detergents.

416 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 2O 10 20 10 3O 20 10 , , i c 2 3 4 GRAMS PER LITRE '5 Graph A B C Soil Deter gent Water Fat only Nansa L Soft Natural Sulphated alcohol Soft Egg/fat Nansa L Hard