8 COTTON WOOL AS AN AIR FILTER. 



a bright ray, for example, from the electric light, or lime 

 light, is thrown across a darkened room. When the flame of a 

 spirit-lamp is then placed under this ray it gives the appearance of 

 smoke passing through it, but there is no smoke from the spirit- 

 lamp, and the black space is produced by the heat of the lamp burn- 

 ing the particles floating in the luminous beam, and for the time 

 being rendering that part void, or empty of particles. The black spot 

 thus produced is said in scientific phrase to be " optically empty." 

 The experiment may be turned to practical and really useful 

 account, by showing us that these particles may be prevented from 

 entering the lungs. Thus, a handful of cotton is placed against 

 the mouth and nostrils, and a full breath inhaled through it, which 

 is easily done. The cotton is now removed, and the air in the 

 lungs made to pass through a glass tube into the luminous ray, 

 when a dark smoke-like space is seen, as in the previous experi- 

 ment with the spirit-lamp. This shows that the air is filtered of 

 its particles by passing through the cotton. 



DISTINCTIVE CHARACTERS OP INFECTIOUS DISEASES. 



Accepting, as we do, the theory that each case of infectious 

 disease originates in the reception of a distinctly specific, pre- 

 existing poison, and that it in turn becomes self-propagat- 

 ing, we now go on to speak a little in detail of each of these 

 zymotics. There are some features which are common to the 

 whole group. They all, for example, begin with a period of what 

 is called "dormancy" or "latency" or "incubation," during which 

 the poison is actively developing. But the duration of this 

 period differs in each case. That of smallpox is twelve days ; 

 typhus fever, eight to fourteen days ; typhoid fever, fourteen to 

 twenty-one days ; scarlet fever, three to six days ; measles, about 

 fou^paays. These differences in the length of the incubation 

 period being probably due in each case to the amount and 

 strength of the poison received. At the termination of this 

 period, the sickness is said to begin, although its distinctive 

 character may not appear for some days longer. These fevers 



