This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
While the special seat of action is the first marked fact about the pathogenetic properties of drugs, the special kind of action is the second fact. This may be seen in the sensations and modalities of a drug. Thus the burning pains of Arsenic, the coldness of Camphor and Veratrum, the sticking pains of Bryonia, the stinging pains of Apis and Theridion, the plug sensations of Anacardium, the soreness of Arnica and Hamamelis, are all characteristic. Frequently the character of these pains indicates the seat of the action, and thus points to the elective affinity of the drug, as burning pains in general indicate the mucous membranes; dull, boring, gnawing pains, the bones; sticking, cutting pains, serous membranes; etc. In many drugs these conditions may be so expressive of their special character, that we nearly always expect them to be present when they are the homceo-pathically indicated, and, therefore, prove to be the curative remedy. Such characteristic conditions are the restlessness and anxiety of Aconite and Arsenic, the chilliness of Pulsatilla, the thirstlessness of Apis, the dullness and drowsiness of Gelsemium, the hysterial contradiction of its symptoms of Ignatia, the melancholy of Aurum, etc.