22 M. MELLONI ON THE FREE TRANSMISSION 



of every thousand rays emanating from the source, each screen transmits 

 or stops tlie following quantities : 



Order. Rays transmitted. Rays stopped. 



1. 484 516 



2. 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 0-01 2 (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 verj' 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 striae 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 



