air during its process of recovery. By the adoption of such means, large wounds can be made to heal, without the formation of a drop of “pus” or “matter.” (See also page 110.) =Suspended Matters= are _mineral_ or _organic_, the two being commonly associated together. The _mineral matters_ consist largely of fine particles of common salt, silica, clay, iron rust, dried mud, chalk, coal, soot, and similar substances. Not uncommonly the mineral particles are coated by, or mixed with, organic matter, the comparative lightness of the organic matter enabling the mineral matter to float about more easily. The objection to dust is thus intensified, for not only is it irritating to the respiratory passages and generally disagreeable, but it carries with it putrescent and possibly morbific particles. The prevention of infectious diseases resolves itself largely into means for preventing the inhalation of dust. =Organic Suspended Matters= in the =open air= are, most commonly, minute fragments of wood and straw, dried horse litter, fragments of insects, the spores and pollen of plants, and microscopic plants and animals. In addition, there is the putrescent organic matter resulting from respiration and other organic functions. =Indoors=, the air commonly contains, in addition, fragments of cotton, linen, silk, or other fibres, fragments of vegetables, starch cells, soot, charred wood, splinters from floors, etc. =In Sick Rooms=, products of the morbid conditions may be evolved; thus, pus-cells, particles from the expectoration, blood cells, fat particles, epithelium, or the special germs or microbes to which infectious diseases are due. These are disturbed by the movements of persons, causing the dust to rise; and thus the infection of consumption, and of the acute infectious diseases, is frequently spread. Flies and other winged insects are important auxiliaries in the diffusion of disease-carrying particles. Receiving some morbid secretions on their limbs, or other parts of their bodies, they have occasionally been the means of spreading erysipelas in hospitals, and glanders in veterinary stables. The specific contagia of cholera, enteric fever, and summer diarrhœa are occasionally conveyed to food by flies which have previously alighted on latrines or privies or other places where the stools of such patients have been deposited (page 281). The excreta of flies, which are not uncommonly deposited on food, or on articles of furniture, have occasionally being found to contain the minute ova of intestinal worms. =Effects of Suspended Matters.=—The inhalation of dust is followed by deleterious effects. We may divide the solid substances inhaled as dust into three kinds:—dead substances, living substances, and the contagia (microbes or germs) of various diseases.