SiGERsoN — On the Atmosphere. 
21 
longer the same adhesive power, and may be blown or shaken off. When 
they have lain on the glass for a little time, the homogeneous mucus will, 
in certain cases, commence to resolve itself into these arborescent forms, 
until we have, as in fig. 1 , the appearance of a number of fern leaves. 
In other cases, as in figs. 2 and 3, the formation may not become so 
distinctly dendritic ; in others, as in portions of fig. 4, in 5, and in 
6, this arborescence may not at all occur. They might be divided into 
three classes : 
In the first class, as shown in figs. 1, 2, and 7, the mucus crys- 
tallizes out more or less completely into beautiful arborescent forms. 
To the presence of muriate of ammonia this is due ; it is known to 
exist in some of the animal secretions, and its crystallization forms are 
characteristic. 
These dumb bell forms may have emanated from the lower animals 
or from man. That represented in fig. 2 of the photomicrographic Plate 
was caught whilst passing through a cattle show, and, doubtless, was 
emitted by some lowing ox. 
On the other hand, that shown in fig. 7 undoubtedly came from 
man, as it was captured at a public assemblage. 
The mucus which has crystallized out is likely to be innocuous. 
In the second class we have particles of mucus, which, as shown in 
fig. 5, present the appearance of ridges and cavities; but whilst remain- 
ing on the glass these do not alter in shape, except as the result of be- 
coming drier : the cavities are produced generally by air-bubbles, but 
sometimes some fatty or oily matter would seem to be present. 
In the third class, as shown in the lower part of fig. 4, and in the 
whole of fig. 6, we find the mucus neither remaining constant nor 
crystallizing out, bat resolving itself into granules. Such granules 
readily separate from the glass, and, being blown about, might, if cap- 
tured — their source being unknown — be mistaken for ''germs" of some 
kind or other. 
Granules of a like kind, but appearing separately, were observed in 
the atmosphere of a fever patient. As I believe them to be able to 
serve as the physical basis of contagion, let me state what occurred. The 
patient was a child who had been prostrated with infantile remittent 
fever of a typhoid type ; after a severe sickness, he was recovering. 
At this time I had passed throngh an attack of coryza ; and though the 
cold was over, there was some remaining delicacy of the membranes of 
eyes and nose. At separate occasions it was forced upon my attention, 
on visiting the convalescing fever patient, that there was some matter 
in his atmosphere to make me sneeze, my eyes to water, and the 
gravedo to return. There was a mere momentary impression of fever- 
ishness of the skin. This occurred on distinct occasions, and I con- 
sidered it might be of service to make a micro-examination of this air ; 
a great multitude of somewhat transparent granules were observed, 
and I have come to the conclusion that these were exudation granules ; 
and that when strength begins to return, and they are exuded and 
breathed forth into the atmosphere by the patient, they form that 
