379 
“ A MESSAGE FEOM THE STARS.” 
we possess in relation to this subject. Having occasion to 
transmit a gas through stoneware tubes surrounded by burning 
fuel, he discovered that the tubes were porous, and that the gas 
escaped outwards into the fire, while at the same time the 
gases of the fire penetrated into the tube. Priestley does not 
appear to have had any idea of the value of this observation : 
it was to him a barren fact. Dr. Dalton perceived the im- 
portant indication, and clearly saw the bearing of Priestley’s 
observation on the properties of aerial bodies. Experiments, 
well devised and cautiously varied, led him to the discovery 
that any two gases, allowed to communicate with each other, 
penetrated each other, or mixed, in opposition to the influence 
of their weight. Taking the lightest known gas, hydrogen , 
and the heaviest, carbonic acid , he placed them in two vessels 
which communicated with each other, so that the dense gas was 
in the lower vessel. According to the solicitation of gravity, 
the two gases should have remained as they were arranged ; but 
it was found that the lighter gas descended and the heavier one 
ascended, until, in a few hours, they became perfectly mixed. 
But for the operation of this diffusive power, the healthful 
condition of the Earth’s atmosphere would not be maintained. 
When a light and heavy gas are mixed , they do not exhibit any 
tendency to separate on being allowed the most perfect repose. 
Common air is essentially a mixture of oxygen and nitrogen : 
these gases differing in weight in the proportion of 971 to 1105. 
Yet if a closed tube of air, many yards in length, be kept 
upright and perfectly still for months, no change whatever takes 
place, the air at the top and that at the bottom of the tube 
being in precisely the same condition. If, however, into a 
similar tube we pour a heavy gas, and then carefully float on it 
a light one, they will have diffused thoroughly in a few hours, 
every portion of the tube containing the same mixture. 
Mr. Graham found that gases diffuse into the atmosphere, 
and into each other, with different degrees of ease and rapidity. 
His mode of observing this was by allowing each gas to diffuse 
from a bottle into the air through a narrow tube, arranged in 
such a manner that the gas had no tendency to flow out , but 
was compelled to diffuse in opposition to the effect of gravity. 
Each gas penetrates into the space occupied by the other, not 
at the same rate in both directions, but according to a well- 
determined law. To express this disposition — or rather diversity 
of disposition — in gases to interchange particles, the term 
diffusion volume has been adopted. The diffusion volumes are 
inversely as the square root of the densities of the gases, and 
the times of effusion and diffusion follow the same law. 
It has been found convenient to adopt with precision the 
following terms : effusion, or pouring out — this expresses the 
