142 Mr. Ritchie on the permeability oj transparent 
in the white of an egg, and a very delicate transparent liquid 
screen will be formed. Place the screen between the dif- 
ferential thermometer with cylindrical chambers, (a descrip- 
tion of which has already been laid before the Society*) and the 
heated body, and the following facts will be observed. 
ist. Raise the ball to a low temperature, keep the screen 
almost at the same temperature, by constantly applying the 
white of an egg mixed with cold water, to the upper side of 
the frame, and no sensible effect will be observed on the 
instrument. 
2nd, Raise the ball to the temperature at which it has just 
ceased to be visible in the dark ; place it at a greater distance, 
and a very striking effect will immediately be observed. 
This experiment clearly proves that radiant heat freely 
permeates a very thin transparent liquid screen. I also find 
that heat begins to radiate through this screen when the ball 
is at a lower temperature than what is necessary to make it 
radiate through a screen of glass, or in other words, a liquid 
screen is more permeable by radiant heat than a solid one. 
Experiment 4. 
Place the screen at different distances from the heated ball, 
and very little difference will be observed in the descent of the 
fluid. In one experiment, the effect was 18 degrees with 
the screen close to the instrument, and the fluid rose only 
one degree when the screen was removed five inches towards 
the heated ball. 
Professor Leslie has demonstrated that when the heat is 
absorbed by the screen, and radiated from its posterior sur- 
* Phil. Trans. 1827, p. 129. 
