he death rate from infectious diseases,
especially such as whooping cough, measles, scarlet fever, diphtheria,
small-pox, and typhus. The infection of all these diseases is
communicable through the air, and where there is overcrowding, the chance
of being infected by infective particles, given off by the breath or
skin, is of course very great. Where there is overcrowding, the
collections of putrescible filth are multiplied, and with them probably
the productive foci of infective particles. Tubercular disease, common
sore throat, chicken-pox, and mumps, are also among the diseases which
are increased by overcrowding.

To come to details which are more specific, let us consider the case of
some diseases which are definitely caused by floating matter in the air.
First, let us take one which is apparently attributable to pollen.

HAY FEVER.

Among diseases which are undoubtedly caused by floating matter in the air
must be reckoned the well-known malady "hay fever," which is a veritable
scourge during the summer months to a certain percentage of persons, who
have, probably, a peculiarly sensitive organization to begin with, and
are, in a scientific sense, "irritable."

This disease has been most thoroughly and laboriously investigated by Mr.
Charles Blackley, of Manchester, who, being himself a martyr to hay
fever, spent ten years in investigating the subject, and published the
result in 1873, in a small work entitled "Experimental Researches on the
Causes and Nature of _Catarrhus aestivus_ (hay fever or hay asthma)."

Mr. Blackley had little difficulty in determining that the cause of his
trouble was the pollen of grasses and flowers, and his investigations
showed that the pollen of some plants was far more irritating than the
pollen of others. The pollen of rye, for example, produced very severe
symptoms of catarrh and asthma, when inhaled by the nose or mouth. Mr.
Blackley came to the conclusion that the action of the pollen was partly
chemical and partly mechanical, and that the full effect was not produced
until the outer envelope burst and allowed of the escape of the granular
contents.

Having satisfied himself that pollen was capable of producing all the
symptoms of hay fever, Mr. Blackley next sought to determine, by a series
of experiments, the quantity of pollen found floating in the atmosphere
during the prevalence of hay fever, and its relation to the intensity of
the symptoms. The amount of pollen was