22 M. MELLONI ON THE FREE TRANSMISSION 
of every thousand rays emanating from the source, each screen transmits 
or stops the following quantities : 
Order. Rays transmitted. Rays stopped. 
1. 484: 516 
Die 380 620 
3. 303 697 
By means of these data we obtain as the values of the calorific losses, 
considered with reference to the quantities of rays which present them- 
selves successively to pass through the three equal layers into which we 
may suppose the last screen divided, 
0°516 0°215 0°203. 
These losses are still greater than those preceding, because of the 
badness of the material and the greater thickness of the layers, but they 
are still in a decreasing progression. Thus the diminution continues 
beyond 54 millimetres. 
To compare this diminution with that which took place in the last 
screen in the preceding experiments we must multiply 0°012 (the differ- 
ence between 0°215 and 0°203) by 2:068, and divide the product by 27. 
In this way we obtain the mean diminution for a thickness of 2™"-068 
in passing from 54 to 81 millimetres, which is nearly 0-001; in the pre- 
ceding experiment it was fifteen times as much while the rays passed 
through the same layer of 2™™-068 placed at a distance of 6 millimetres. 
The difference would be still greater if we had used very transparent 
layers of glass, such as flakes of the glass of a mirror attenuated. 
Nevertheless I had some doubts as to the homogeneity of the glass: 
I was afraid that the striz might not be equally distributed over all the 
points of the mass. But not being able to procure large pieces of this 
material entirely free from defects, I thought that analogous experi- 
ments performed with liquids might answer quite as well. In employ- 
ing these instead of glass there was, in case of success, the additional 
advantage of extending the law of calorific transmission by making it 
independent of the physical constitution of the medium. 
I procured therefore several copper troughs, of the same breadth but 
of different lengths, bounded at each end by a glass plate. These I placed 
successively between the perforated screen and the pile in such a manner 
that the anterior glass plate was quite near the screen, the distance of 
which remained constantly the same. The common section of the troughs 
was much larger than the central aperture of the screen; the reflexions 
on the lateral faces could not take place, and the only rays that entered 
a little out of the perpendicular direction reached the anterior surface of 
the pile. The lamp was moved up so near that the needle of the gal- 
vanometer exhibited a deviation of 30° through the two glass plates of 
each trough. The radiation was then intercepted, the trough filled with 
