BOOK NOTICES. 699 
DUST AND FOG.—BENEFICIAL EFFECTS OF SMOKE. 
Mr. John Aitken recently read a paper before the Royal Society of Edin- 
burg on the origin of fogs, mists and clouds. From a great number of experi- 
ments with moist air at different temperatures, to determine the conditions which 
produce condensation of water vapor, he concludes that whenever water vaj or 
condenses in the atmosphere, it always does so on some solid nucles; that dust 
particles in the air form the nuclei on which the vapor condenses; that if there 
were no dust there would be no fogs, no clouds, no mists, and probably no 
rain; and that the supersaturated air would convert every object on the surface 
of the earth into a condenser on which it would deposit as dew; lastly, that our 
breath, when it becomes visible on a frosty morning, and every puff of steam, 
as it escapes into the air from an engine, show the impure and dusty state of 
the atmosphere. These results have been verified at temperatures as low as 
14° Fah., at which, however, there was little cloudiness produced, owing to the 
small amount of vapor in air socold. The sources of this dust are many and 
various; for instance, finely ground stone from the surface of the earth, the ash 
from exploded meteorites, and living germs. Mr. Aitken showed experimentally 
that, by simply heating any substance, such as a piece of glass, iron or wood, a 
fume of solid particles was given off, which, when carried along with pure air into 
a receiver, gave rise to a dense fog mixed with steam. So delicate is this test 
that the hundredth of a grain of iron wire will, when heated, produce a distinct 
haziness in the receiver. By far the most active source of these fog-producing 
particles is, however, the smoke and sulphur given off by our coal fires; and as 
even gas grates will not prevent the emission of these particles, Mr. Aitken 
thinks it is honeless to expect that London, and other large cities wherein such 
fuel is used, can ever be free from fogs. However, inasmuch as more perfect 
combustion will prevent the discharge of soot flakes, these fogs may be rendered 
whiter, purer, and therefore more wholesome, by the use of gas grates, such as 
that recomended by Dr. Siemens. Mr. Aitken also drew attention to the deodor- 
izing and antiseptic powers of smoke and sulphur, which, he thinks, probably 
operate beneficially in killing the deadly germs and disinfecting the foul smells 
which cling about the stagnant air of fogs, and suggests caution lest, by suppress- 
ing smoke, we substitue a greater evil for a lesser one.—Sczentific American. 
BOOKNOTICES: 
NATURAL SCIENCE AND RELIGION, by Asa Gray. pp. 111, octavo. Chas. Scrib- 
ner’s Sons, N. Y., 1880, $1.50. 
This little work is made up of two lectures delivered by Prof. Asa Gray to 
the Theological School of Yale College; the first upon Scientific Beliefs, and the 
