August 2, 1889.] 



SCIENCE. 



71 



worked out very perfectly in every detail, and a high degree of 

 mechanical skill is shown in their construction. 



The armature-shafts are of high-grade steel. The bearings are 

 all accurately fitted, and are very long in proportion to their diame- 

 ter, being, in the smaller sizes, of hard composition, and in the larger, 

 of babbitt-metal. The commutators, which ordinarily are liable to 

 great wear and damage, have received particular attention, being 

 made of a special hard bronze. All the motors are provided with 



Fig. s. 



switches for starting and stopping, and in the larger sizes the 

 switches are provided with resistance-coils, — an arrangement 

 which is much handier than a separate rheostat. 



In respect to simplicity, all parts needing attention, being in plain 

 sight, are easily accessible. The armatures may be removed for 

 inspection or any other purpose, and replaced in running order, in 

 less than one minute. All parts are made to standard gauges, and 

 are interchangeable. 



CONSIDERATIONS CONCERNING SOME EXTERNAL 

 SOURCES OF INFECTION IN THEIR BEARING ON 

 PREVENTIVE MEDICINE.^ 



No department of medicine has been cultivated in recent years 

 with such zeal and with such fruitful results as that relating to the 

 causes of infectious diseases. The most important of these results 

 for preventive medicine and for the welfare of mankind is the 

 knowledge that a large proportion of the causes of sickness and 

 death are removable. 



It is evident that efforts to preserve health will be most intelli- 

 gently and effectually applied when they are based upon an accu- 

 rate and full knowledge of the agencies which cause disease. 

 Public and private hygiene, however, cannot wait, and fortunately 

 has not waited, for the full light of that day, whose dawn has only 

 begun to appear, when we shall have a clear insight into the causa- 

 tion of preventable diseases. Cleanliness and comfort demand 

 that means shall be taken to render pure the ground on which we 

 live, the air which we breathe, and the water and food with which 

 we are supplied ; and we must meet these needs without waiting 

 to learn just what relation infectious agents bear to the earth, air, 

 water, and food. 



It is a fortunate circumstance that modern sanitation has been 

 controlled so largely by the belief in the dependence of endemic 

 and epidemic diseases upon organic impurities in the soil and in 

 the water. Incomplete and even erroneous in many respects as 

 are the views which have prevailed concerning the origin and 

 spread of epidemic diseases by the decomposition of organic sub- 

 stances, the sanitary measures which have been directed toward 

 the removal of filth have achieved great conquests in limiting the 

 development and extension of many infectious diseases. The 

 benefits which one commonwealth of this country has derived from 

 the intelligent employment of public sanitary measures were clearly 

 and forcibly presented before this association last year by Dr. 

 Walcott, in his admirable address on State medicine. 



While nothing should be said, or need be said, to lessen the 

 importance of cleanliness for public health, it is important to bear 

 in mind that hygienic cleanliness and esthetic cleanliness are not 

 identical. In water which meets the most severe chemical tests of 

 purity, typhoid bacilli have been found. On the other hand, the 

 air in the Berlin sewers, which certainly does not meet the most 



1 Address in State medicine, delivered before the American Medical As 

 in Newport, on Friday, June 28, by William H. Welch, M.D., professor of pathology 

 in Johns Hopkins University, Baltimore. 



modest demands of esthetic cleanliness, has been found to be 

 nearly or quite free from bacteria. 



It needs only to be stated to be generally admitted that the sci- 

 entific basis of preventive medicine must be the accurate knowledge 

 of the causative agents of preventable diseases, — a knowledge 

 which can be derived only from a careful study of all of the prop- 

 erties of these agents, the modes of their reception and of their 

 elimination by the body, the circumstances which favor and those 

 which retard or prevent their development and spread, their behav- 

 ior in the various substances which surround us or which we take 

 into our bodies, and the sources of infection, not only those which 

 laboratory experiments show to be possible, but those which are 

 actually operative. 



So long as we were unacquainted with the living organisms 

 causing infection, the means at our disposal for studying the etiol- 

 ogy of infectious diseases were limited to the observation of all of 

 the circumstances which we could determine regarding the origin 

 and spread of these diseases. We could only infer what might be 

 the properties of the infectious agents from the study of phenomena 

 often obscure and difficult of interpretation. Chiefly by this method 

 of investigation the science of epidemiology has been built up. It 

 has established facts and laws no less of practical than of scientific 

 importance ; but it has left unsolved many problems, and has filled 

 gaps with speculations. Admitted epidemiological facts are often 

 open to various interpretations. 



We are evidently at a great advantage when we can study the 

 epidemiological facts with a knowledge of the substances which 

 actually cause infection, and this we are now enabled to do for a 

 limited number of the infectious diseases. This new method of re- 

 search, which thus far has been mainly bacteriological, has aided 

 us not so much by simplifying the problems of etiology, which still 

 remain complicated enough, as by affording greater accuracy to 

 the results. 



It is my aim in this address to consider some results of the mod- 

 ern studies of pathogenic micro-organisms in their bearing upon 

 preventive medicine, more particularly upon the sources of infec- 

 tion. It is, of course, impossible within the limits of the address 

 to attempt a complete survey of this important field. Time will 

 permit the presentation of only some of the salient points. 



Infectious diseases are those which are caused by the multipli- 

 cation within the body of pathogenic micro-organisms. 



It has always been recognized that some infectious diseases, 

 such as the exanthematous fevers, are conveyed directly from the 

 sick to the healthy. It is not disputed that in these evidently con- 

 tagious diseases the infectious germ is discharged from the body 

 in a state capable at once of giving rise to infection. 



In a second group of infectious diseases, of which malaria is the 

 type, the infected individual neither transmits the disease to another 

 person, nor, so far as we know, is capable of infecting a locality. 

 Here there is reason to believe that the infectious germ is not 

 thrown off in a living state from the body, but is destroyed 

 within the body. In this group the origin of infection under natu- 

 ral conditions is always outside of the body. 



In a third group there is still dispute whether the disease can be 

 transmitted directly from person to person, but all are agreed that 

 the infected individual can infect a locality. It is especially fortu- 

 nate that the bacteria which cause cholera and typhoid-fever, the 

 two most important representatives of this group of so-called 

 miasmatic contagious diseases, have been discovered and isolated 

 in pure culture. These are the diseases about whose origin and 

 epidemic extension there has been the greatest controversy. They, 

 above all other diseases, have given the impulse to public sanita- 

 tion during the last half-century. The degree of success with 

 which their extension in a community is prevented is an important 

 gauge of the e.\cellence of the local sanitary arrangements. A 

 clear comprehension of the origin and spread of these diseases sig- 

 nifies a solution of many of the most ve.xed and important prob- 

 lems of epidemiology and of State hygiene. 



It is difficult to understand how those who accept the discovery 

 that the bacteria causing typhoid-fever and cholera have been 

 found and cultivated from the stools of patients affected with these 

 diseases can doubt that these patients are possible sources of con- 

 tagion, or can entertain the view, once so widely prevalent, that the 



