He knows that relatives usually kill each other by means of particular poisons; that other poisons are used for suicidal purposes; that the photographer takes cyanide of potassium, the medical man and chemist prussic acid or morphia, the poor man vermin-killer or oxalic acid, or carbolic acid, or some such agonising destroyer of life. And thus, though all poisons lead to the same end--stoppage of the breathing and blood circulation--yet each has its own particular way of sending the soul to eternity. He can therefore often tell the analyst detective how to take a short cut. [Illustration: THE SPECTROSCOPE--AN INSTRUMENT THAT HAS BEEN FATAL TO MANY CRIMINALS.] By the way, there is no such thing as a slow poison--that is, a poison which, taken to-day, does not show its effects for weeks. This is a fiction of the novelists. On the other hand, there is--except in the case of prussic acid and nicotine--no death straight away after taking poison, as one sees it on the stage, Shakespeare notwithstanding. An actual case will show that the discovery of murder by the doctor and analyst is not always plain sailing. A good many years ago, a Mr. Sprague was tried for the murder of the Walker family by means of the well-known poison of the deadly nightshade. The medical evidence showed clearly that they all died from belladonna poisoning, and belladonna was found in the rabbit-pie they had for dinner. A common-sense jury, however, acquitted the prisoner; and only recently have medical men solved the mystery by discovering that rabbits can eat any quantity of this plant without suffering harm, while their flesh becomes fatally poisonous. A second case shows what wonders the chemists can work. A surgeon's wife died from corrosive sublimate, given in a draught by her husband. He said that, in making up the draught, he mistook a bottle of mixture, which he had prepared for a sailor, for the water-bottle, and had poured some of it into his wife's draught. The sailor's mixture was analysed, and it certainly contained corrosive sublimate; but, not content with finding the poison, the analyst measured the quantity present, and, while the sailor's mixture contained only ten grains to an ounce of liquid, the wife's draught contained fifteen grains, showing that the surgeon's ingenious explanation was a lie! Blood is so characteristic a fluid that it might be supposed a skilful analyst could never have any difficulty in recognising it. Of course, if he were given, say, a cupful in its ordinary state, he could not make a mistake. But he never gets a chance of earning his fee so easily. When the police seek his assistance they give him, perhaps, a suit of dirty clothes, which may be stained by two or three small dark spots that might be anything.