VIII.] RAPE DUST 241 



used as food, mustard oil is generated in the stomach 

 to a dangerous extent. It thus becomes the custom 

 only to use the purer grades of rape cake for cattle 

 feeding ; in the other cases the cake is ground to 

 powder and sold for manure. More recently a method 

 of extracting the ground rape seed with carbon 

 bisulphide, in which the oil is soluble and can be 

 recovered by distilling off the solvent, has been gener- 

 ally adopted, because the whole of the oil in the seed 

 is obtained in this way. The residue, which is really 

 improved by the complete removal of the oil, is only 

 used for manure. 



Rape dust, as the ground rape cake is termed, has 

 long been valued as a manure; William Ellis in 1735 

 speaks of oil cake with approval as one of the Hertford- 

 shire " hand dressings " for corn, and at the time of the 

 beginning of scientific agriculture in the second quarter 

 of the last century we find that the use of rape dust 

 had become pretty general throughout the eastern 

 counties. 



Rape dust contains about 5 per cent of nitrogen, 

 with such small quantities of phosphoric acid and potash 

 that it must in the main be treated as a nitrogenous 

 manure. In its action it may be classed with the fish 

 and meat guanos previously described, in that decom- 

 position and nitrification is set up pretty rapidly and 

 continues throughout the whole season. It has been 

 largely used in the Rothamsted experiments, and the 

 results with barley and mangolds (Tables XXVI., XXX., 

 and LXIII.) show that, nitrogen for nitrogen, it is 

 almost as effective as nitrate of soda or sulphate of 

 ammonia. In these cases, however, the manure is 

 applied year after year to the same land, so that the 

 residues unused in the year of application accumulate 

 for the benefit of the crop in future years, other 



Q 



