152 SCIENCE AND FRUIT GROWING 



smeared his trees with vaseline. Yet, this point appears never 

 to have been considered in this country, and only to a limited 

 extent elsewhere, chiefly in the direction of comparing the action 

 of " crude oil " with that of " kerosene," which, in the absence of 

 details as to the composition of the oils used, can afford but 

 little guidance. 



So far as undiluted paraffins are concerned, their use in this 

 country has practically been abandoned. In applying them in 

 a diluted form, the oil and the water were originally sprayed on 

 to the trees simultaneously, through separate jets, and it was not 

 till 1878 that emulsions of kerosene and soap were recommended 

 to the public (by Cook, in America), although it is probable that 

 the " mixtures " used by George Cruikshank, of Whitinsville, 

 Mass., since 1870, were in reality emulsions. 



The part played by the soap in an emulsion had been assumed 

 to be merely a physical one, and the proposal has recently been 

 made to use flour, scorched flour, or lime, as a substitute for it. 

 But the Woburn results show that the soap in the emulsion has 

 an appreciable insecticidal action, and that the proportion of 

 it used is by no* means a matter of indifference. 



The multiplicity of recipes recommended in this country for 

 making emulsions, renders it evident that they have been for- 

 mulated on little or no principle, and are not the outcome of 

 actual investigation. A casual search in the literature of the 

 subject brought to light 24 different recipes for emulsions, 

 the strongest containing 80 times as much paraffin as the 

 weakest, with the proportions of paraffin to soap showing a 

 200-fold variation. Of the 24 recipes, no less than 13 were 

 issued by the same authority the Board of Agriculture 

 only to be withdrawn within a year or two of their issue 

 (VI, 19). It is, no doubt, necessary that emulsions of dif- 

 ferent strengths should be used for different purposes and 

 under different conditions, but research has not been so complete 

 yet as to show that every insect requires a special strength of 

 emulsion, or that its requirements vary from year to year; and 

 the only conclusion must be that such recommendations were 

 based on the mere fancies of those advocating them, and not on 

 the requirements of the case as determined by direct experiment. 



Some attempt has been made at Woburn to remedy this state 

 of confusion, by a careful examination of the action of various 

 emulsions, as well as of undiluted paraffin and other substances, 

 on the mussel scale, of which a more detailed account will be 

 found in Chapter XX : at the present moment attention may 



