Weaiher, Water and Disease. 17 



nrsT AND For,. 



We come now to what may be called accidental features of 

 our air. Fir.st, its dust and fog. Our present knowledge 

 of it has been obtained by scientific and precise processes ; we 

 can literally count the floating dust of our atmosphere. 

 Tyndall's electric light demon.strated its abundance and its 

 presence everywhere. Aitkin has invented recently an in- 

 strument for counting the infinitely small atoms. We shall 

 give the resi:lts of his investigations, and especially as they 

 bear on our sanitary state {A^a/z/rc, Nov. 5, i8gi, p. 10, etc. 

 Dr. W. J. Russel, at the Hygienic Congress). His basic 

 principle is that a change of state, a gas passing to a liquid 

 or a liquid to a solid, really always occurs at what he terms a 

 "free surface;" that as long as a molecule of liquid water 

 is surrounded by like molecules, and the .same with gaseous 

 water, we do not know at what temperature, or whether at 

 any, they would change their state ; but if in contact with a 

 solid then at the surface, where they meet, the change will 

 occur. The dust always in the atmosphere offers this free 

 surface to the gaseous water, and thus induces its condensa- 

 tion. This specific action of dust varies very considerably. 

 First, 'with regard to its composition, and second, with regard 

 to the abundance and size of the particles present. Sulphur 

 burnt in the air is a most active fog-producer, so is salt. 

 Many hygroscopic bodies form nuclei having .so great an 

 affinity for water that they can cause its condensation from an 

 unsaturated atmosphere.. At the same some, non-hygroscopic 

 bodies, such as magnesia, and many others, are powerful fog- 

 producers. 



The products of combustion, even when the combustion is 

 perfect, are powerful fog-producers. He shows how exceed- 

 ingly small an amount of matter is capable of inducing fog. 

 One one-hundreath of a grain of iron wire, however, after it 

 was heated, involved on each heating sufficient dust to cause 

 a visible fog, then with still more delicate apparatus, that 

 tt/oo of a grain of either copper or iron gave a similar result, 

 and he infers that 'even tWttuo of a grain would yield 

 sufficient nuclei to cause visible fog. This small amount of 

 dust can be filtered out, and then no fog will occur. Mr. 

 Aitken has invented an instrument, small enough to go into 



