16 M. MELLONI ON THE FREE TRANSMISSION 



timetres in length, blackened on the inside; and at a certain distance 

 from the mouths of these tubes are placed the stands destined to receive 

 the screens. In strictness, a single tube and a single stand would be 

 sufficient, and one of the sides of the pile might be closed by means of 

 a small metallic cover ; but, when we have to operate on bodies differing 

 in quality and thickness, it often happens that they differ in tempera- 

 ture not only from one another but from the pile also. Then if we 

 place but one screen before the apparatus, the calorific actions at the 

 two sides are unequal, the index of the galvanometer moves away from 

 zero, and we must wait for some time until the equilibrium of the tem- 

 perature is established and the index returns to its original position. 



Now this inconvenience cannot occur when the pile is furnished with 

 two tubes and two stands ; for, by placing before each side of it a plate 

 of the same quality and thickness, it is clear that, if care be previously 

 taken to place the two in the same circumstances, they will have the 

 same temperature, and will consequently emit the same quantity of heat 

 on the two sides of the pile. The index of the galvanometer will re- 

 main stationary, whatever may be the difference of temperature between 

 the plates and the thermoscopic body, and we may therefore immediately 

 proceed with the experiments. Hence, if we would save time, we should 

 always have a pair of screens of each sort; and, as we have just observed, 

 put both sides of the pile in the same state. 



In order to ascertain the influence exercised on free transmission, by 

 the different circumstances relating to the surface, the volume, and the 

 composition of the screens, we must procure a constant source of heat. 

 For this purpose, there is nothing better than a good lamp with a double 

 current of air and a constant level. When this apparatus is well pre- 

 pared and filled with oil freed from mucilage, by means of sulphuric acid, 

 we obtain a flame which maintains an invariable temperature for more 

 than two hours. Of this I have been able to satisfy myself by means of the 

 thermomultiplier. But in order to have things in this preparatory state, 

 we must wait some moments until the pipe, the oil, and the glass funnel 

 of the lamp shall have attained a maximum of temperature. This time, 

 which ^'aries a little with the construction of the lamp, is about ten or 

 fifteen minutes. 



There may be some objections raised against the employment of an 

 Argand lamp as a calorific source. It will be said, perhaps, that in this 

 lamp the heat acts only through the glass funnel ; that the funnel itself 

 becomes heated, and mixes its rays of nonluminous heat with the lumi- 

 nous caloric of the flame ; and lastly, that such a source of heat is 

 neither uniform nur separated from the agent which usually accom- 

 panies it in high temperatures. 



But I wish it to be particularly observed, that the only thing about 

 which we are interested at present is, to know ^\hether the state of the 



