It may be pointed out that bones have done much to alter our system of farming, by helping to develop turnip culture. Used at first in comparatively large pieces, experience gradually showed that a finer state of division facilitated their action. Yet it was long before the prejudice in favour of rough bones disappeared; and it was not till 1829 that Mr Anderson of Dundee introduced machinery for preparing 1/2-inch and 1/4-inch bones and bone-dust. In the early days of their use, bones were fermented before being used, in order to render their action more speedy when applied to the soil; and this practice still obtains to the present day in some parts of the country among farmers. This fermentation was often effected simply by mixing the bones with water, and allowing the heap to lie for a week or two. In other cases the bones were mixed with urine or other refuse matter. The most important step, however, in the history of the treatment of bones for manure was the discovery in 1840, by Liebig, of the action of sulphuric acid on them--a discovery which led to the institution of the manufacture of superphosphate of lime by Sir John Lawes. The nature of this action will be explained in the following chapter, so that we need only say here that the efficacy of the manure by treatment with sulphuric acid is more than doubled. Bones have thus been used, and still are used, in a variety of conditions, such as in the raw or green state, bruised, boiled, steamed, fermented, burned, dissolved, and broken or ground into various states of fineness, to which the names of 1/2-inch, 1/4-inch bones, bone-meal, bone-dust, and floated bones are given. We shall now proceed to discuss the composition of bones, and investigate more exactly the nature of their action. _Composition of Bones._