EEESH lift. 
15 
As for the injury done by other gases, that is so little and 
so exceptional that I need hardly refer to them. Wherever 
sulphuretted, phosphuretted, or carburetted hydrogens appear, 
they are indicative of the presence of other matters in the air 
more injurious than themselves. I shall not therefore dwell 
on them, but turn to the solid particles which render the air 
impure, and with which these gases are often associated. 
These solid particles are so minute that they can only be 
apprehended by the microscope, and many of them, even by 
that instrument, are not sufficiently made out to be easily 
distinguished. They are derived from organic or inorganic 
sources. The organic are derived from living or dead animals 
and plants. The particles thus given off are exceedingly 
minute, and appear to be held in suspension by molecules or 
small particles of water. The emanations of living animals 
are constant. The epidermis of the skin flies off into the air, 
as well as particles from the lungs in the breath, so that the 
air where large numbers of animals exist becomes charged 
with such exhalations. The human body is no exception to 
the law. These particles are capable of decomposition, and 
when taken again into the living system, may be absorbed 
and lead to febrile disturbance in the system. These particles 
are given off from diseased bodies in such a state that they 
generate diseases in other bodies like those from which they 
have come. It is in this way that zymotic diseases are pro- 
pagated, and scarlet fever, small-pox, measles, hooping-cough, 
and typhus, are all conveyed in this way. 
Dead animal matter gives off also particles, not equally 
destructive of life, but which may, nevertheless, produce the 
most virulent diseases. Typhoid fever is the offspring of 
decomposing animal matter. The particles • which produce it 
steal up from our drains and cesspools, and make their way 
into the studios of the scholar and the chambers of royalty ; 
no class or condition of persons are spared the influence of 
this dreadful poison. 
Vegetable matter decomposing emits still more destructive 
poisons. The malaria of our own marshes, and its deadly 
representative in the Campagna of Rome, with the miasma 
escaping from the swamps of Africa and the jungles of Asia, 
are all of vegetable origin. Plants decomposing in contact 
with water yield this dreadful agent, which contaminates and 
renders deadly the purest of atmospheres. 
Another set of particles which may come from animal, 
vegetable, or mineral sources, are those which we call dust. 
Dust is not only unpleasant — it is dangerous to life. The 
workers in coal are liable to disease in the lungs, from the 
particles of coal-dust accumulating in the lungs and producing 
