The above summary is by no means perfect, since there are minor differences in each class, which may sometimes rise into such prominence as to confuse the classification. But in medical evidence on the individual poisons of which we treat, those physiologically resembling them in action are always most heard of at the trial, and questions are asked whether this or that may not produce the same symptoms; and hence it is well to direct attention to the analogues of our types. The primary idea of an _alkaloid_ is derived from its resemblance to an _alkali_. Alkaloids are often called also “Organic Bases.” Their names terminate in—_ia_ or _ine_.[2] They are more or less alkaline to test paper, and combine with acids to form salts which are neutral in reaction and often crystallizable. Only a few of the alkaloids are liquid and easily volatile, but almost all can be volatilized by careful heating at definite temperatures, giving in many cases a sublimate of characteristic appearance under the microscope, either of crystals, globules, or a mere film. In a free state, the alkaloids are very slightly soluble in water, but soluble in alcohol, and generally in ether and chloroform. Some are soluble in benzine, others in amylic alcohol, petroleum spirit, acetic ether, &c. On a judicious use of these various solvents depend the different processes of isolation, among which Dragendorff’s is the most complete, but so complicated that it is rarely used in its entirety. Fortunately there is generally a clue more or less definite to the probable poison administered, enabling a shorter and quicker method to be adopted. For further details as to these processes see Blyth’s Manual of Practical Chemistry. The sulphates, chlorides, and acetates of the alkaloids are generally soluble in water; if ammonia or potash be added to the solution, a precipitate (usually crystalline) of the free alkaloid occurs if the solution be of moderate strength. Chemically, the alkaloids are derived from ammonia (NH_{3}) by substituting various organic groups or “compound radicles” (compounds of carbon and hydrogen), for the hydrogen of the ammonia. They are therefore “compound ammonias,” or “amines.” Nitrogen, carbon, and hydrogen, are always present in natural alkaloids, the non-volatile ones, including the greater number, also contain oxygen.