The greater part of these were attacked in the five days January 26-30,
1881, and in seeking for the source of infection of these cases, special
attention was directed to the time about a fortnight previous viz.,
January 12-17, 1881. The comings and goings of all who had been directly
connected with the hospital (ambulances, visitors, patients, staff,
nurses, etc.) were especially inquired into, but with an almost negative
result, and Mr. Power was reluctantly forced to the conclusion that
small-pox poison had been disseminated through the air.

During the period when the infection did spread, the atmospheric
conditions were such as would be likely to favor the dissemination of
particulate matter. Mr. Power says: "Familiar illustration of that
conveyance of particulate matter which I am here including in the term
dissemination is seen, summer and winter, in the movements of particles
forming mist and fog. The chief of these are, of course, water particles,
but these carry gently about with them, in an unaltered form, other
matters that have been suspended in the atmosphere, and these other
matters, during the almost absolute stillness attending the formation of
dew and hoar frost, sink earthward, and may often be recognized after
their deposit.

"As to the capacity of fogs to this end, no Londoner needs instruction;
and few persons can have failed to notice the immense distances that
odors will travel on the 'air breaths' of a still summer night. And there
are reasons which require us to believe particulate matter to be more
easy of suspension in an unchanged form during any remarkable calmness of
atmosphere. Even quite conspicuous objects, such as cobwebs, may be held
up in the air under such conditions. Probably there are few observant
persons of rural habits who cannot call to mind one or another still
autumn morning, when from a cloudless, though perhaps hazy, sky, they
have noted, over a wide area, steady descent of countless spider webs,
many of them well-nigh perfect in all details of their construction."

A reference to the meteorological returns issued by the registrar-general
shows that on the 12th of January, 1881, began a period of severe frost,
characterized by still, sometimes foggy, weather, with occasional light
airs from nearly all points of the compass. This state of affairs
continued till January 18, when there was a notable snow storm, and a
gale from the E.N.E. For four days, up to and inclusive o