OF RADIANT HEAT THROUGH DIFFERENT BODIES. 27 



tlie first infinitely thin layer of the liquid ; but this layer, while it is 

 becoming hot, undergoes a certain dilatation, becomes lighter than the 

 rest of the fluid mass, and ascends immediately to the upper part of the 

 vessel, whence it can have no longer any influence on the pile. It is 

 replaced bj"^ a second layer, which undergoes a similar process, and this 

 again by others ; so that by these partial renovations of the liquid 

 screen, the hinder part of the glass applied to the aperture of the tube 

 is not in contact with heated molecules, and retains the same tempera- 

 ture for a long time. 



It was extremely difficult to make flat glass vessels with very regular 

 surfaces of the same thickness throughout, and with the opposite sides 

 exactly parallel. Metallic frames and glasses joined with gum could 

 not be employed because of the corrosive action of the several liquids. 

 After many a fruitless effort to surmount this difficulty, I thought at last 

 that the process by which the index of refraction of liquids is measured 

 in optics might be available in this case also. With this view I had 

 quadrangular pieces of two centimetres in breadth and nine centi- 

 metres in length cut out of several pieces of the same mirror unsil- 

 vered and sufficiently thick. I laid close to the two faces of each of the 

 pieces from which the excision had been made two flakes made out of 

 another and a much thinner glass. It is known that the mere adhesion 

 of two plates of polished gljiss is sufficient to prevent the passage of 

 liquids. However, in order to be more secure, I introduced each reci- 

 pient between two metallic frames, which held the thin glasses in their 

 places by means of four screws placed at the angles. The liquid was 

 poured into these vessels at a small aperture made at the top, and fur- 

 nished with a glass stopper. In such a system there could be no doubt 

 of the parallelism of the faces and the equal thickness of the layers. 



The results furnished by the several bodies, both solid and liquid, I 

 have disposed in several tables, each of them exhibiting at the top the 

 common thickness of the screens employed and, beside the substance, 

 the indications of the thermomultiplier and the quantity of rays trans- 

 mitted as compared with the whole radiation. This distribution, while 

 it allows the use of plates of different thicknesses, has the additional ad- 

 vantage of presenting distinct groups of each class of bodies. The free ra- 

 diation in each case was 30°. In order to link the results of these tables 

 together, I have commenced the second and the third with the numbers 

 given by afiake of glass placed in the same circumstances as the plates 

 which constitute each group : thus the glass set down in the table of 

 liquids was contained between the two thin plates of the recipients, and 

 made of the thick looking-glass employed in their construction. It was 

 therefore exactly of the same thickness as the liquid layers, and, like 

 them, came into contact with the thin plates which formed the faces of 

 the recipients. But as those faces themselves intercepted a portion of 



