22 



SCIENCE. 



[Vol. YL, No. 124. 



stances, it is usually very difficult, and often 

 impossible, to obtain precise knowledge by 

 mere observation of results, even when such 

 observation can be made accurately. The 

 most rapid and satisfactory progress is made 

 when we can subject the problem to the test of 

 experiment, and, by varying the conditions at 

 our pleasure, can thus determine those which 

 are essential to any particular result which may 

 appear. 



This experimental method is now being ap- 

 plied to sanitary questions, and especially to 

 those connected with the prevention or sup- 

 pression of the infective or spreading diseases, 

 which are those of most interest in public 

 hj'giene. We can now study the causes of 

 splenic-fever, er3'^sipelas, infectious pneumonia, 

 tuberculosis, glanders, and probably cholera, 

 in the laboratory as well as in the sick-room ; 

 their causes may be cultivated, like plants, 

 and the effects of various foods, temperatures, 

 etc., upon them determined ; and the efficacy of 

 means proposed for eradicating them may be 

 tested by direct experiment. 



It is true that as 3'et comparatively few dis- 

 eases can be thus investigated ; because for the 

 majority we have not yet found any animals, 

 other than man, who are susceptible to them, 

 and we cannot use man for inoculation experi- 

 ments. We cannot say whether we have pres- 

 ent in a specimen of water or a piece of clothing 

 the specific cause of tj^phoid-fever, or of yel- 

 low-fever, or of scarlet-fever, except by tests 

 applied to man ; and hence we can only surmise 

 with more or less probabilit}^ as to whether 

 such causes exist in a given well, or ship, or 

 bale of rags. Nevertheless, the progress has 

 been so great during the last five years, that 

 we have ever}' reason to hope that science will 

 before long be able to use her right hand 

 (experiment) to aid her left hand (observa- 

 tion) in unravelling not a few of these tangled 

 skeins. 



It is no longer a satisfactor}" explanation of 

 an outbreak of diphtheria or typhoid to say 

 that the place was filthy : if that were a suf- 

 ficient cause, there are few towns that would 

 not soon suffer from epidemics of these dis- 



eases. The apparent anomalies in the distri- 

 bution of disease, which are apparent to every 

 one who has investigated the subject, — the 

 good health of persons who work in the midst 

 of offensive effluvia and typical filth, the prog- 

 ress of an epidemic along one side of a street 

 while the other side is free from disease, the 

 people who drink dilute sewage with impunity 

 and enjoyment, — all these things are illustra- 

 tions of our own ignorance, and not of varia- 

 bility in natural law, to which all alike must be 

 subject. 



For every person who is affected with ty- 

 phoid, or cholera, or tubercle, there are fifty 

 who, so far as we can see, have been exposed 

 to the same causes, and remain unaffected. 

 It is easy to find instances where children have 

 slept with others aflTected with scarlet-fever 

 without contracting the disease. We cannot 

 say to a person, " If you let your child visit his 

 playmate sick with scarlet-fever, he will con- 

 tract the disease : " we can only sa}^ that it is 

 very probable that he will do so. In like 

 manner, we cannot say to the inhabitants of a 

 town or village where the wells are within a 

 few yards of leaky privy-vaults, "You will have 

 typhoid or cholera : " the probabilities may be 

 onl}^ one out of four or out of ten that this will 

 happen. 



Sensible people take some precautions when 

 their cattle are in such danger : they insure 

 their houses and barns against much smaller 

 probabiUties of loss, but they have not yet 

 learned that it also pays to insure against 

 disease. 



"Aiis instruction must be given them, not in 

 the form of spasmodic declamation and vague 

 threats, but as clear, definite information, dis- 

 tinguishing carefully between that which is 

 known, or sanitary science, and that which is 

 only more or less probable ; and scientific in- 

 vestigators, whether chemists, geologists, biol- 

 ogists, or physicians, must all aid in the work. 

 Their special knowledge gives them power, 

 and it also imposes on them responsibility, — 

 a responsibility which, if neglected, may result 

 in crippling their chosen work and filling their 

 own homes with sorrow. 



