50 METRICAL JURISPRUDENCE grey powder, magnesium carbonate, white lead, red lead or ferric oxide will develop them. Finger prints on paper, wood and textile fabrics may be successfully developed by treating them with 5 per cent silver nitrate solution and then fixing them with sodium thiosulphate.52 Finger prints on paper may also be developed by exposing it to the vapours of iodine or osmium tetroxide and by brushing the surface with some coloured solution. The prints developed with the aid of iodine vapours are fugitive, and should, therefore, be photo- graphed at once. Mitchell53 suggests the application of osmium tetroxide by exposing the prints to the vapour of a boiling one per cent solution of this reagent in water. The colour solution may be writing ink of any colour or some dye dissolved in water or alcohol. A solution of osmium pyrogallate prepared by mixing 2 cc. osmic acid and 0.05 gramme pyrogallic acid in 2 cc. water gives satisfactory results.54 Major Henry Smith, I.M.S., has discovered that it is possible to forge thumb impressions by covering the original thumb impressions with a piece of damped paper and pressing, by which method the reverse of the original is transferred to the damped paper. Another piece of damped paper is then put over the reverse and pressed, and a true copy of the original is thus obtained.55 7. FOOTPRINTS The impression of a foot or a boot left on the ground in the vicinity of the place of occurrence of a crime has often led to the arrest of the criminal. To identify the footprint a fresh footmark of the suspected person should be obtained and compared with the original. During the examination a careful note should be made if there are any peculiarities in the foot, such as flat foot, scars resulting from wounds, or callosities, as these are likely to be found in the footprint if it is well marked. In the case" of a bootmark the peculiar arrangement of the nails, or holes in the sole may be useful in comparing with the original. It is often said that a footprint made by an individual while he is walking is smaller than the one made by him while he is standing, but I have found from experiments that a footprint produced in walking is generally larger than the one produced in a standing position. It is usually assumed that the impression left on the material composed of loose particles, such as sand, is smaller than the foot or boot producing it, while the impression on mud, clay or some material not composed of freely moveable particles, is larger. Casts of footprints may be taken by spraying the print with a'rapid- drying fixative, such as an 80 per cent alcoholic solution of shellac or a 4 per cent solution of cellulose acetate and then smearing it with a thin film of lubricant, e.g. a mixture of mineral oil and melted lard. The print is afterwards surrounded by a wall of cardboard or wood, about 1J inches high. An aqueous mixture of plaster of Paris of the consistency of ordinary cream is gently poured on the print within the enclosed area. After 10 to 15 minutes the cast is set completely, but it is desirable to let it stand for a further period of 15 minutes before it is removed. The setting of plaster of Paris may be hastened by dissolving salt in the water used, or may be retarded by adding 7 per cent of acetic acid or a little calcined lime to the water.56 52. Henry T. F. Rhodes, Forensic Chemistry, 1941, p, 10. 53. The Analyst, XLV, 1920, pp. 122-29 ; Lucas, Foren. Chem. and Scientific Investi- gations, Ed. m, p. 184. 54. Ibid., p. 185. 55. Ind. Med. Gaz., June 1902, p. 255. 56. Bulletin, Bureau of Criminal Investigation, New York State Police, Sep. 1942; Med. Leg. and Criminolog. Rev., Vol. XI, Part IV, 1943, p. 196.