The period at which the disease shows itself after the exposure of an individual to sources of contagion, is exceedingly various. One child will be seized within a few hours; another, not for some days; and now and then (though rarely), five or six weeks have intervened between the period of exposure and the manifestation of the disease. When this disease is rife in a family, it will frequently affect the individuals composing it very differently. Some escape altogether;-- others have the mild form of the complaint;--others the severe;--and, again, the attendant in the sick room may be attacked with the sore throat and fever only, both of which may subside without any appearance of a rash. In conclusion, this disease is a complaint of infancy and childhood, rather than of adult age; generally affects the same individual but once during his life; and, though examples of a second attack have occurred, such a circumstance is extremely rare. Sect. VI.--MEASLES. Measles consists of a fever, in which the mucous lining of the air- passages is principally affected, and which, after about three days' duration, results in an eruption of a red rash over the surface. It depends upon a specific contagion;--occurs most frequently during childhood and adolescence, though no age is exempt from it;--and affects the system but once; a peculiarity to which an exception is very rare, proved by the few instances of the kind which have been recorded. The period at which the disease manifests itself after infection is various,--generally about the ninth day; it has, however, been delayed until the sixteenth. DESCRIPTION OF THE DISEASE.--The child infected will be observed not to be as well as usual, less active, and out of spirits; his appetite will fail, and his sleep be restless and disturbed. It will soon be evident that he has apparently taken a cold in his head, and that this is accompanied by fever. His voice will be hoarse; there will be frequent cough, headach, sneezing, running from the nose and eyes,--the eyelids being somewhat swollen, and the eyes inflamed;--the skin will be hot and dry, and he will complain of occasional chilliness. In the course of the next two or three days, these symptoms will increase in severity, and perhaps be accompanied by oppression at the chest and hurried breathing, and towards evening by slight delirium.