60 PRESERVATION OF FOOD BY REFRIGERATION ; 
sun and at 29° 8’ in the shade, and an hour later 114° in the sun 
with a shade temperature of 32° 6.” 
Again, in the polar regions in the summer time the pitch 
has been observed to run from the seams in the vessel’s 
side, while the thermometer in the shade close by has stood 
below freezing. Had the air the power of absorbing 
radiant heat to any appreciable extent, it is quite evident 
that such differences in temperature as those above cited 
could not have occurred. 
But while dry air is thus shown to be almost perfectly 
transparent to heat rays, the aqueous vapour suspended 
in it acts in a very different manner. The following experi- 
ment of Tyndall’s may also be mentioned as showing the 
difference between the absorptive power of dry air and 
aqueous vapour. ‘Testing dry air for its absorptive power, 
he first experimented with a vacuum. A long tube was 
mounted with rock-salt ends, and the air from the interior 
withdrawn by means of an air pump. A body radiating 
heat at a constant temperature of 212° Fahr. was then fixed 
in such a manner that the heat rays emerging from it and 
passing through the rock-salt ends of the tube fell upon a 
galvanometer at the other end. This was carefully adjusted 
and the number of degrees the index moved was noted. 
Dry air was then admitted, and the heat rays, as before, 
passing through the rock-salt ends, had now the air to 
traverse before falling upon the galvanometer. A move- 
ment of less than 1° was perceptible, the index then 
remaining stationary. The dry air was then withdrawn by 
means of the air pump, the galvanometer again noted, and 
the ordinary atmospheric air from the room allowed to fill 
the tube. Although the amount of aqueous vapour in the 
air was only 45 per cent. of the amount capable of being 
carried at the temperature noted, a deflection 72 times as 
great as that from the dry air was produced, thus showing 
that the aqueous vapour suspended in the air exercised an 
absorptive power more than 72 times as great as that of the 
air itself. 
Loss or Heat sy Rapiation.—‘‘1. Let P (in fig. 1, plate IT) 
be a block, say, of building stone, 1 foot cube, having its surfaces, 
SS, as also the air in contact with it, maintained at a constant 
