144 Intelligence and Miscellaneous Articles. 



be taken of introducing into the tube containing the mercury a 

 small quantity of concentrated sulphuric acid to protect the terminal 

 surfaces of the mercury from oxidation. 



In order to employ this apparatus for the study of the pheno- 

 mena of radiant heat, a polished brass cone is placed inside at a 

 short distance from the bulb, the diathermanous substances to be 

 investigated being fixed between the small aperture of the cone and 

 the bulb ; the heat of a wax taper, sent back by a small reflector, 

 suffices for most experiments. The absorptive and emissive powers 

 of the various substances are easily studied by preparing a certain 

 number of small disks of copper (such as the circular copper plates 

 of a Tolta pile) ; each disk is coated on one side with lampblack, 

 on the other with the substance whose absorbing-power is to be de- 

 termined. All the disks being heated together in a metal vessel, 

 we have only to place them successively at the same distance from 

 the bulb, the substance to be studied facing the bulb, in order to 

 determine the emissive powers. 



If the same disks be placed all at the same distance from a stove 

 or any source of heat, the different faces being turned towards the 

 source of heat, and be then placed successively each with its black 

 face looking towards the bulb, the inequality of their absorbing- 

 powers is shown by the inequality of the deflections of the instru- 

 ment. 



As will be seen, the instrument is nothing else but a differential 

 bulb-thermometer ; but its indications are more easily read at a dis- 

 tance than those of Eumford's or Leslie's apparatus. Of course, as 

 the apparatus varies with the barometric pressure and the tempe- 

 rature of the surrounding medium, the horizontality of the lever 

 must be established, by shifting the cursor, before commencing a 

 series of experiments. 



This apparatus may be made of very various dimensions. That 

 which I use has its lever about 14 centims. in length, the mercury 

 index 5 centims. I have constructed another much smaller, of only 

 3 centims. length, the bulb of which has a diameter of only about 

 5 millims., and the tube is almost capillary. With the instrument it 

 is very easy to verify the distribution of the heat in the solar spec- 

 trum. It can also be constructed with two bulbs, like the ordinary 

 differential thermometer ; but it is then less sensitive*. — Biblio- 

 theque Universelle, Archives des Sciences Physiques et Naturelles, 

 tome iv. no. 7, pp. 71-73. 



* At the time I constructed the above-described instrument I knew 

 nothing about M. Marey's thermograph (see Methode graphique dans les 

 sciences experimentales, par M. E.-J. Marey, p. 314). This appears to me 

 more difficult to construct than that which I propose ; but it has the 

 advantage of possessing a fixed bulb. 



