A witness engaged to give expert evidence should demand his fee before going into court, or, at all events, before being sworn.
With regard to notes, these should be made at the time, on the spot, and may be used by the witness in court as a refresher to the memory, though not altogether to supply its place. All evidence is made up of testimony, but all testimony is not evidence. The witness must not introduce hearsay testimony. In one case only is hearsay evidence admissible, and that is in the case of a _dying declaration_. This is a statement made by a dying person as to how his injuries were inflicted. These declarations are accepted because the law presumes that a dying man is anxious to speak the truth. But the person must believe that he is _actually_ on the point of death, with _absolutely_ no hope of recovery. A statement was rejected because the dying person, in using the expression 'I have no hope of recovery,' requested that the words 'at present' should be added. If after making the statement the patient were to say, 'I hope now I shall get better,' it would invalidate the declaration. To make the declaration admissible as evidence, death must ensue. If possible, a magistrate should take the dying declaration; but if he is not available, the medical man, without any suggestions or comments of his own, should write down the statements made by the dying person, and see them signed and witnessed. It must be made clear to the court that at the time of making his statement the witness was under the full conviction of approaching or impending death.
It is but seldom that medical evidence is required with regard to the identification of the living, though it may sometimes be so, as in the celebrated Tichborne case. The medical man may in such cases be consulted as to family resemblance, marks on the body, nævi materni, scars and tattoo marks, or with regard to the organs of generation in cases of doubtful sex. Tattoo marks may disappear during life; the brighter colours, as vermilion, as a rule, more readily than those made with carbon, as Indian ink; after death the colouring-matter may be found in the proximal glands. If the tattooing is superficial (merely underneath the cuticle) the marks may possibly be removed by acetic acid or cantharides, or even by picking out the colouring-matter with a fine needle. With regard to scars and their permanence, it will be remembered that scars occasioned by actual loss of substance, or by wounds healed by granulation, never disappear. The scars of leech-bites, lancet-wounds, or cupping instruments, may disappear after a lapse of time. It is difficult, if not impossible, to give any certain or positive opinion as to the age of a scar; recent scars are pink in colour; old scars are white and glistening. The cicatrix resulting from a wound depends upon its situation. Of incised wounds an elliptical cicatrix is typical, linear being chiefly found between the fingers and toes. By way of disguise the hair may be dyed black with lead acetate or nitrate of silver; detected by allowing the hair to grow, or by steeping some of it in dilute nitric acid, and testing with iodide of potassium for lead, and hydrochloric acid for silver. The hair may be bleached with chlorine or peroxide of hydrogen, detected by letting the hair grow and by its unnatural feeling and the irregularity of the bleaching.