X-ray absorption and X-ray diffraction analysis of bone substance:​ ground sections of compact and spongy bone of several mammals of different age were used. The distribution of minerals in the bone tissue is not uniform and depends partly on the relative proportion of collagen and osteomucoïd of the matrix. The Ca content of the primary periosteal bone is always higher (5 to 20%) than that of the secodnary bone, e.g., Haversian systems. The calcification of the newly laid down bone increases slowly and progressively, the increment being more rapid in the matrix which lies close to the vascular channels, more rapid in youn individuals than in adult and old subjects. The three varieties of bone tissue (fibrous, parallel fibered and lamellar bone) show the same X-ray diffraction patterns regardless of the degree of calcification of the matrix and of the age of the bone material.