At the commencement of the last century, a certain physician 130 attributed the cause of almost all diseases to microscopical insects, and gave figures of ninety species which were supposed to produce, in some cases smallpox, in others rheumatism and gout, jaundice and whitlows. Almost all these figures represent imaginary creatures. This opinion has reappeared in modern times; how many persons have been seen to smoke camphor in order to preserve themselves from the invasion of animalcules. I do not speak of the apparatus which has been contrived in order to breathe nothing but air which has been filtered and deprived of its living germs. There are some of the articulata with four pairs of feet, a kind of microscopic spiders which require to be noticed here; these are the numerous Acari which infest many animals. Some of these wander on the surface of the skin, others in galleries under the epidermis, and many pass from one animal to another without changing their form or mode of life. There is a considerable number of them; no class of the animal kingdom is free from them, neither aquatic nor terrestrial animals, neither vertebrates nor invertebrates. These parasites belong for the most part to the same family, and cause by their presence a disease which was for a long time considered to be peculiar to the skin. An English naturalist, Mr. George Johnson, carefully studied the parasitical and free acaridæ of Berwickshire. Mons. Ehlers has written a very interesting work, with fine illustrations, on the acaridæ of birds, published in the "Archives of Troschel." There is more than one species which lives at the expense of man, and one of them produces 131 a disease known in every country and at all times under the name of the itch; until 1830 its true nature was still unknown. It is not an affection of the skin, as was thought, but merely the result of the presence of these animalcules. The director of the special Hospital for Skin Diseases at Paris was so fully convinced that the acaridæ are not the cause of the itch, that he offered a prize to any one who could render these insects visible. A student of medicine, a Corsican 132 by birth, had happened to see these itch-insects sought for in his own country, and was the first to prove, in 1834, the real cause of the disease. A resident student had given, in a thesis which he sustained at Paris before the faculty of medicine, a drawing of a cheese-mite instead of the itch-insect, and this error had caused it to be supposed that the species peculiar to this disease did not exist. We give in Figures 21, 22, 23, representations of the male and female insect, greatly magnified.[2] Of course, all the treatment necessary for the cure consists in getting rid of the animalcules and their eggs, and in cleansing the skin and the clothes of the patient. Petroleum oil has been judiciously prescribed in order to destroy the