The third great division of the physical sciences is physics; and here, too, the contributions have been indispensable. For what would public health work amount to if there were no microscopes or other optical electrical and mechanical apparatus devised by physicists? How helpless the health official would be without the physical knowledge which he now employs in many of his major health undertakings: water purification, waste disposal, illumination, heating, ventilation, the pasteurization of milk. Again, the physics of the radiant spectrum is already playing an important part in diagnosis and therapeutics - both of which have important health applications - and promises a brilliant future of usefulness. The science of physical chemistry, according to the prognostications of many students of public health, is destined to make some of the most fundamental of all contributions to both theory and practice. It must be remembered that with the development of modern conceptions of the atom, the claim of chemistry to finality regarding the composition of matter has been challenged, and in so far as this challenge may be sustained, the more fundamental and far-reaching discoveries will be expected from the province of physics. That medicine and public health will share in these discoveries cannot be doubted.

And finally, in discussing the scientific basis of public health work, it is necessary to remember the intimate relation existing between that activity and what are called the medical sciences. Several of these have been referred to in passing, and it only remains to discuss relationships. The division of all science into subjects is, of course, only a convenience. It is understood that all experimentally verifiable knowledge forms a consistent whole, and that its classification into different disciplines is chiefly an acknowledgment of the limitations of the individual mind. If there were such a thing as a composite human mind, there would be little need of such classifications, for it would be able to follow the electron into the atom, the atom into the molecule, the molecule into protoplasm, the latter into the millions of known forms of life with all the various ramifications of their physiological processes, and the relations of these forms to each other and to their environment, without being conscious of the need for a change of subject on the way. And thus the difference between medical science and practice and public health work is only one of viewpoint and practical convenience. As has been stated, in some countries the medical practitioner is essentially the effective public health agent. He not only treats his patients for the diseases for which they consult him, but also advises them in hygienic living, administers protective inoculations, supervises bedside precautions to prevent the spread of disease, maintains isolation of contagious cases, and is otherwise active in carrying out a number of health precautions. In the event of an epidemic he becomes an important factor in control measures.

In this country, however, the attitude of the practitioner, at least until recently, has been that his public responsibilities were largely ended when he had reported cases of communicable disease occurring in his practice to the official health authority. Hence a sharper distinction arises here between medical and public health activities than necessity demands. The doctor's field has been predominatingly treatment - the health worker's, prevention.

This is not the place to discuss the advantages of the two systems further than to remark that the first, using the practitioner as the health agent, appears to work well in the small, compact, homogeneous countries where it is found, while the second system accomplishes comparably good results under the conditions which prevail widely in this country.

Under our system it is possible for the practitioner to fulfill his obligations without any extensive knowledge of, or experience in, public health work. On the contrary, it is not possible for a responsible health officer to perform his official duties without a thorough grounding in medical science, or at least the assistance of those of his staff" who have had such training.

In summing up this section it may be said that public health work, at least of an official character, is definitely committed to a policy of practice based upon information derived from the basic physical sciences: biology, chemistry, and physics. Without this information it would, in all probability, be somewhat worse than useless. While a part of the data comes directly from the academic sources of scientific information, a large bulk is derived from those adaptations which are known as the medical sciences, and still another portion is the result of researches carried out by health agencies themselves.

It is the intent here to deal with public health agencies only to the extent that is necessary in order to afford an understanding of the outlook of public health work. The observations will be confined to conditions in the United States.

First of all, the agencies may be separated into two classes, the official and the voluntary. Official health agencies are appointed and maintained under law for the benefit of all the citizens of the areas concerned, and their expenses are defrayed out of the taxes. Their employees are responsible to the people for proper performance of duties. Their field usually comprises the whole range of health problems arising in their jurisdictions. They have legal authority to use police powers in the enforcement of health laws. Voluntary health agencies may be subject to local laws regarding incorporation, but are not responsible to the general public. They may engage in general health activities, but are more commonly devoted to one activity or one group of activities, as, for example, the control of cancer or the improvement of health in children. They sometimes apply only to selected groups of the population, such as the students of a university or the employees of a factory. The funds for voluntary health work do not emanate from the tax receipts, but are secured by bequests, donations, various propaganda devices, or the solicited or required contributions of the benefited clientele. They have no legal power to enforce measures.