Intelligence and Miscellaneous Articles. 143 



These results show that the vapour-density of iodine, calculated 

 with a= 0*00367 and PV=1, diminishes quite as much at a low as 

 at a high temperature. 



All the hypotheses which have been framed on the assumption 

 of either a dissociation of, or isomeric change in, iodine henceforth 

 appear to me hardly admissible. In the present state of our know- 

 ledge, nothing authorizes us to suppose that a partial vacuum would 

 be adequate to produce a modification of that nature. The only 

 necessary consequences of the experiments made at high tempera- 

 tures or at low pressures are, that the expansion-coefficient of 

 iodine varies with the temperature, and that its coefficient of com- 

 pressibility varies with the pressure. Every hypothesis proposed 

 in order to explain these results will have to take into account this 

 double variation. — Gomptes Mendus de VAcademie des Sciences, July 5, 

 1880, t. xci. pp. 54-56. 



NOTE ON A DEMONSTRATION- DIFFERENTIAL THERMOMETER. 

 BY HENRI DUFOUR. 



Eor the purpose of exhibiting, in lectures on physics, the prin- 

 cipal phenomena due to the radiation of heat, the thermoelectric 

 pile and a galvanometer are usually employed. If the latter is a 

 reflecting one (such as that of M. Wiedemann), it is easy to render 

 visible to a numerous auditory some of the most delicate thermal 

 phenomena. The only inconvenience of these two instruments is 

 their high price ; perhaps it is on account of this that the study of 

 the radiation of heat is so often neglected in colleges which possess 

 but slender resources. It is in order to render the study of these 

 phenomena possible to every one, that I have constructed the fol- 

 lowing instrument, which can be easily executed anywhere at a very 

 moderate price. 



A tube in the shape of a very widely open V (the two branches 

 making an angle of about 140°) is terminated at one of its ends by 

 a blackened bulb. A horizontal lever of very light wood unites the 

 two branches as the bar of an inverted A (y) would do ; this lever 

 turns on a horizontal axis fixed to the middle of its length ; upon 

 the axis is a vertical needle, which moves in front of a graduated 

 dial, likewise vertical. 



A short column of mercury is introduced into the tube so that it 

 occupies its lower portion. Equilibrium being established, the in- 

 dicating needle is at the zero of the graduation. Under these con- 

 ditions any heating of the bulb produces expansion of the air which 

 it contains, and consequently a displacement of the mercury index, 

 under the influence of which the apparatus inclines more or less ; 

 it afterwards returns to zero when the action of the source of heat 

 ceases to operate. 



To regulate the horizontality of the beam, a small brass cursor 

 can be placed on the lever at a variable distance from the axis. 

 Lastly, the motion of the apparatus is very regular if the precaution 



