520 THE POPULAR SCIENCE MONTHLY. 



air will not convey the jDoison, although it is safe to say that the 

 area of its diffusion by this means is very limited. Instances in which 

 sewer-gas has been known to be the means of conveyance are numerous, 

 but the cases were confined to one or more rooms directly communicat- 

 ing with the sewer, or to a limited part of a building. But the typhoid- 

 fever poison is not a gas. In such a shape we can not conceive that it 

 will lie dormant in suitable soil, that it will undergo phases which so 

 closely resemble germination and growth, or that it can be transmitted 

 to long distances when given a proper vehicle. We have here a right 

 to make a scientific use of the imagination. We can not imagine it a 

 gas, but we can form an idea of it as an atom, a germ capable of pres- 

 ervation, growth, and infinite multiplication. It has never been seen ; 

 it may be that it will never be seen. In the physical sciences we pro- 

 ject ourselves ideally into the midst of many things unseen, yet with 

 perfect theoretical conviction in the reality of their existence. 



In the group of cottages to which the epidemic was confined we ob- 

 serve a certain order in the arrangement of the infected buildings. A 

 close examination of the plan is nearly sufficient to convince one that 

 this order is not due to chance. Nor can we explain it by a conveyance 

 of the disease-germs in the clothing of the inmates. We have in the 

 exempt houses a sufficient warrant against this supposition. The in- 

 habitants of the exempt houses, as the outbreak of the fever extended, 

 assumed generously nearly all the care of the afflicted families. They 

 were daily and nightly in attendance upon the sick, and in many cases 

 assisted in washing the linen. If this were true of the inhabitants of 

 the houses that were exempt from the scourge, we are nearly safe in 

 excluding personal conveyance of the germs by those who inhabited 

 houses showing a sequence of infection. 



During my attendance upon several of the cases I repeatedly looked 

 \)ver the ground and studied the habits of the difi"erent families, but, I 

 am ready to confess, without discovering the clew to the extension of 

 the disease from the single focus in the person of Schmidt ; that is, to 

 discover a clew that would meet the demands of the scientific germ the- 

 ory. However, I became convinced that if I were able to find some 

 domestic arrangement or prevailing condition that linked the infected 

 families together under a common liability to the disease, I should be- 

 come master of the position. Such a connecting link I found in the 

 water-supply of the different families. I did not make the discover}^ 

 accidentall}^, but only as a result of a systematic investigation. In all 

 researches of this kind I believe that it is rare that the truth is stumbled 

 upon in the course of a careless search. If we do not exclude cause 

 after cause in the course of the search, we are almost sure in the end 

 to halt midway between two probable conclusions without being sure 

 of either. 



It is necessary to recall the fact that five houses, g, h, i, e, and d, 

 were supplied with water from the well t — an open curb-well, loosely 



