to the public health will be considered in detail in special chapters. Dampness of the soil of a locality is also an important factor, and its influence in relation to consumption has been carefully investigated by Dr. Bowditch, of Boston, and Dr. Buchanan, of London. Their investigations have shown that the death-rate from consumption is in proportion to the dampness of the soil. Dampness of soil is also an important predisposing factor in the production of many other diseases, such as malaria, rheumatism, and catarrhal affections.

The influence of the nature of the occupation, and of the climate of a locality, as predisposing factors of disease will be discussed in detail in special chapters on these subjects.

Sanitary science refers to the investigation of the causes of disease and the means of avoiding or destroying them. It is not a specific department or separate branch of science, but is implied, in part, in a number of sciences, as chemistry, biology, physics, pathology, statistics, etc.

The term sanitary means conducive to the preservation of, and the term sanitory conducive to the restoration of, health. The sanitary condition of a place has reference to the presence or absence of the specific causes of disease. There is no such thing as bad hygiene: A place is either in hygienic condition or it is in an unhealthful condition.

Hygiene aims to discover the causes of all diseases known, and the best means of removing those causes or rendering them inoperative. It takes for granted a knowledge of the normal functions of the human organism, and seeks to discover the reasons for perverted action of a part or the whole of the organism. It involves a thorough knowledge of the normal conditions of man's environment as well as the various factors which tend to render that environment abnormal. It demands a thorough knowledge of the chemical and physical character of man's food-supply and those changes to which it is liable that tend to injure his health and produce disease. It aims to keep persons in perfect health, to train men to

be strong both mentally and physically. It also involves a knowledge of the physical and geological nature of the surface of the earth, and the manner in which these conditions, in different localities, influence the healthfulness of human habitations. It comprises a knowledge of all the various human ocupations, and the manner in which these may be conducted so as to be free from danger to health or how to render them least objectionable.

Hygiene may be subdivided into several departments relating to the scope of its application, as public or general, military, naval, personal, municipal, school, and industrial hygiene. Public hygiene takes cognizance of factors which affect the general public, such as nuisances of different kinds: Foul odors, noxious gases or dust evolved in certain manufacturing processes, and loud noises. Nuisances are generally such conditions which aggravate existing disease rather than produce disease. Military, naval, personal, school, and industrial hygiene will be treated more or less generally in special chapters. Municipal hygiene has reference to those conditions which affect the general health of a community that fall directly under the control of municipal governments, such as the influence of impure water-supplies and imperfect drainage upon the general health; the influence of overcrowding in the habitations of the poor; the cleansing of city streets and the removal and satisfactory disposal of refuse matters; the regulation of the isolation and care of those affected with infectious diseases, and the proper disposal of the dead.

Development of Hygiene.-Modern hygiene has been gradually evolved out of the observations and discoveries of many men prominent in philanthropic work, in medicine, and in science. Among the prominent observations and discoveries made during the eighteenth century. which have been most instrumental in the development of hygiene may be mentioned the discovery of Sir George Baker with regard to the production of leadpoisoning by cider stored in leaden vessels; the observa

tions of John Howard with regard to the baneful influence of foul air and overcrowding and unhealthfulness of the surroundings upon the health of the occupants of prisons, poor-houses, and other habitations, and their relation to typhus fever; the demonstration by Captain James Cook, in his voyage around the world, that scurvy was a preventable disease which was due to the nature of the diet; and Sir Edward Jenner's discovery of inoculation as a preventative of small-pox. During the nineteenth century the movements and discoveries which stand out most prominently are the work of Dr. Thomas Southwood Smith and The Sanitary Committee in demonstrating the factors which are instrumental in influencing the health of towns, such as the accumulation of filth about premises, absence of sewers, and consequently the pollution of water-supplies, and the influence of insufficient air-supply and overcrowding upon the general health; the labors of Edwin Chadwick in organizing the first board of health in England; the work of Dr. William Farr, Registrar-General of England, in securing the registration of the cause of death in the health reports; the labors of Dr. E. A. Parkes in demonstrating the evil effects of defective drainage and the accumulation of filth upon the public health, and in securing the passage of various sanitary acts from 1848 to 1857; the work of Dr. John Simon, of London, and his able staff of medical inspectors with regard to the material causes of disease, and the legislation which was based upon these investigations; the studies of Dr. C. A. Louis, of Paris, upon typhoid, typhus, and relapsing fevers, and the differentiation between these, as well as similar studies made at the same time by Dr. William W. Gerhard, of Philadelphia; the studies of Dr. Henry I. Bowditch, of Boston, and those of Dr. George Buchanan, of London, upon the influence of dampness. of the soil upon the prevalence of consumption; the studies of Louis Pasteur upon the causes of fermentation and the etiologic relation of micro-organisms to disease, as well as his discoveries with regard to the prevention

and treatment of these diseases; the studies of Sir Joseph Lister with regard to the prevention of suppuration in wounds, which have been the starting-point of modern antiseptic and aseptic surgery; the work of von Pettenkofer in introducing new methods of chemical research upon air, water, and food, and his studies upon the influence of soil-moisture upon the prevalence of typhoid fever and cholera; and the discoveries of Dr. Robert Koch of the specific micro-organisms of some of the infectious diseases, and in perfecting methods of bacteriologic investigation.

Since we have become acquainted with the direct causative factors of many of the contagious diseases, we have developed preventive measures that were impossible before the causes of disease were known. The discovery of the influence of different chemical and physical agents, in destroying the bacteria outside the body, has been of very great value in limiting the dissemination of disease. The discovery of immune serums, especially the antitoxins of diphtheria and tetanus, has been of great service in limiting the dissemination of disease when employed both as prophylactic and curative agents. The introduction of the methods of inoculation of attenuated living or dead cultures of bacteria for the prevention and cure of bacterial diseases has been an important step forward, especially in the control of cholera, plague, and typhoid fever. The discovery of the influence of certain suctorial insects in the dissemination of disease, as the several species of mosquitoes in the dissemination of malarial fevers, yellow fever, of filariasis, and of dengue; the rôle played by the rat flea in the dissemination of plague, the tsetse fly in the dissemination of sleeping sickness, and the tick in the dissemination of African "tick" fever, are some of the more important of this class of diseases that can now be controlled more intelligently since the mode of dissemination is known. The introduction of modern methods of water purification and sewage disposal are also important factors in limiting the dissemination of disease. by means that were unknown a quarter of a century ago.

CHAPTER I.
AIR.

Nature and Composition of the Atmosphere.Atmospheric air consists of a mechanical mixture of gases, the relative proportions of which are fairly constant in all parts of the world. It is colorless, odorless, transparent, and is, therefore, invisible and imperceptible when quiescent. It is only when it is itself in a state of motion, or when our bodies are in rapid motion, that we note its presence through the resistance which it manifests. It also possesses weight, and conséquently exerts pressure. At the sea-level, when the temperature is o° C., the normal pressure of the atmosphere is sufficient to support a column of mercury 760 millimeters in height, and amounts to 1033 grams on every square centimeter of surface. The pressure of the atmosphere decreases as we rise above the level of the sea, and increases as we descend below its level.

The several gases composing the atmosphere are not in chemical combination with each other, but exist as a more or less homogeneous mixture. The principal gases in the mixture are: Nitrogen in the proportion of 78.20 parts, by volume; oxygen, 20.76 parts; argon, I part; carbon dioxid, 0.04 part; a trace of ammonia; traces of nitrous and nitric acids; small amounts of ozone; varying proportions of aqueous vapor; and traces of several recently discovered constituents: neon, erythron, and krypton.

The proportion of nitrogen in natural air varies only within extremely narrow limits. It is an indifferent gas, and seems to serve principally as a diluent for the oxygen in the air. So far as known, the only biologic significance of nitrogen is its absorption by plants of the order Leguminosa when growing in symbiosis with certain micró-organisms which find lodgement on the roots of