462 THE POPULAR SCIENCE MONTHLY. 



infectious without being contagious. When, with reference to a case 

 of typhoid fever in his own house, a man asks the question, " Is it con- 

 tagious ? " he does not wish to know whether or not some one in the 

 next street may take the disease, but whether or not there is a likeli- 

 hood of its spreading among the members of his own household, and 

 whether or not there is danger in going near the sufferer. The only 

 accurate and proper meaning of the word is that attached to it in the 

 definition which I have given. That, therefore, is the sense in which 

 it is used in this paper. 



What is the nature of the poisons which j^ass from the sick to the 

 healthy ? Their most distinctive peculiarity is, that they are largely 

 reproduced in the system during the course of the maladies to which 

 they give rise. The minutest possible portion of small-pox matter, 

 for instance, may be introduced into the system of a person who has 

 not had that disease, and who has not been vaccinated, with the cer- 

 tainty of giving rise to a malady during whose course there will be 

 formed many thousand times as much of the poison as sufiiced to set 

 the disease agoing. 



Contagion, then, consists physically of minute solid particles. The 

 process of contagion is the passage of these from the bodies of the sick 

 into the surrounding atmosphere, and in the inhalation of one or more 

 of them by those in the immediate neighborhood. If contagion were 

 a gaseous or vapory emanation, it would be equally diffused through 

 the sick-room, and all who entered it would, if susceptible, suffer alike 

 and inevitably. But such is not the case ; for many people are exposed 

 for weeks and months without suffering. Of two persons situated in 

 exactly the same circumstances, and exposed in exactly the same de- 

 gree to a given contagion, one may suffer and the other escape. The 

 explanation of this is, that the little particles of contagion are irregu- 

 larly scattered about in the atmosphere, so that the inhalation of one 

 or more of them is purely a matter of chance, such chance bearing a 

 direct relation to the number of particles which exist in a given cubic 

 space. Suppose that a hundred germs are floating about in a room 

 containing two thousand cubic feet of air. Th«r-e is one genn for 

 every twenty cubic feet. Naturally the germs will be most numerous 

 in the immediate neighborhood of their source, the person of the suf- 

 ferer ; but, excepting this one place, they may be pi-etty equally dis- 

 tributed through the room ; or they may be very unequally distributed. 

 A draught across the bed may carry them now to one side, now to the 

 other. The mass of them may be near the ceiling, or near the floor. 

 In a given twenty cubic feet, there may be a dozen germs, or there 

 may be none at all. One who enters the room may inhale a germ 

 before he has been in it ten minutes ; or he may remain there for an 

 hour without doing so. Double the number of germs, and you double 

 the danger. Diminish the size of the room by one half, and you do 

 the same. Keep the windows shut, and you keep the germs in ; open 



