Palladius says that the semi-tertian is compounded of a tertian intermittent, and a continual quotidian, the quotidian paroxysm taking place every day, and the tertian every alternate day. He ascribes their origin to a mixture of putrid bile and phlegm. (21.) Haly Abbas gives a similar account of it. He calls it a fever compounded of a tertian and quotidian, and remarks that it has a tendency to pass into hectic. He treats it with diluents and refrigerants. (Pract. iii, 17.) Avicenna gives a very detailed account of the symptoms and cure of the semi-tertians. He remarks that the fever is apt to be protracted and to pass into the hectic. (iv, 1, 4.) See Rhases (Cont. xxxi.) All his authorities agree in describing it as a compound of the tertian and quotidian, having a severe paroxysm the one day and a slighter the following. It is said to be generally protracted to the fortieth day. Platearius, and the other earlier modern writers, describe the semi-tertian in nearly the same terms as the ancients. It is a species of fever still occasionally met with in warm climates. (See Littré’s Hippocrates, t. ii, 569.) SECT. XXXV.—ON EPIDEMIC DISEASES. We call those diseases epidemic and common, that attack many persons together; which, having a common origin, have also a common cause. Common diseases are produced by common food of a bad quality, drinking of bad water, inordinate fatigue, the want of the customary exercise, deprivation or repletion from the prevalence either of a famine or of great abundance. The nature of the country will also often occasion common diseases, either from its lying adjacent to marshes, or to some deep pit, which emits a deleterious and pernicious exhalation. These things are constantly occurring. But the atmosphere which surrounds us may alter the temperaments, by being hotter, colder, or more humid than ordinary. For to other causes we are not all exposed together, nor do we come in contact with them for the whole day; but the ambient air is diffused around all, and is inhaled by respiration. Now, the bodies of animals must undergo a change along with these changes as to temperature. He, therefore, who is acquainted with these matters, will not only be able to predict the diseases which are to arise from every state of the atmosphere, but will be able also to prevent them by substituting a counteracting regimen to the intemperament of the air. Wherefore, those bodies which are disposed to a certain disease, from a peculiar intemperament, will be affected by a similar temperature of the surrounding air. But those of an opposite temperament to the atmosphere will not only not be hurt but will be improved; the excess of their intemperament being overcome by the opposite excess. He that is acquainted with these things will preserve the health, by superinducing the contraries to the constitution of the body; sometimes perhaps using refrigerants, and sometimes heating things, cooling with water, using