238 Dr. Miiller on the Thermal Effects 



coloured liquids. By determining, then^ what portion of the 

 visible spectrum is transmitted by the liquid under examination, 

 I hoped to estimate the thermal action corresponding to this 

 portion. It is, however, of covu*se assumed here as completely 

 established, that the heat- and light-rays are identical, in the 

 sense in which this was supposed to be the case by Masson and 

 Jamin. 



The optical examination of the coloured liquids with which I 

 proposed experimenting, was effected by throwing, by means of 

 a flint-glass prism, a perfect solar spectrum upon a paper screen, 

 introducing the liquid to be examined (contained between two 

 glass plates 15 millims. apart) immediately before the slit 

 through which the solar rays were admitted, and observing, 

 finally, what part of the spectrum remained unextinguished. 



In this manner the following results were obtained : — 



1. A solution of cochineal allowed all the red rays, as far as 

 the commencement of the orange, to pass through perfectly, so 

 that the red of the spectrum was not weakened by the cochineal 

 solution ; but all the remaining colours of the spectrum were 

 completely extinguished. 



2. A solution of bichromate of potash allowed red, orange, 

 and yellow to pass through almost entirely without absorption, 

 together with a trace of green. The entire remainder of the 

 spectrum was completely absorbed. 



3. A solution of chloride of copper absorbs the whole of the 

 spectrum except the green ; but even the green was found to 

 have diminished appreciably in brightness. 



4. A solution of ammonio- sulphate of copper completely 

 absorbed the least refrangible portion of the spectrum, while 

 blue, indigo, and violet were transmitted without appreciable 

 diminution in intensity. 



In order to measure the power of these liquids in absorbing 

 thermal rays, I first allowed the solar rays reflected from a 

 metallic mirror to enter a dark room through a round orifice of 

 1 inch diameter. About 2| metres from this orifice the quadi'atic 

 thermo-battery was placed, so as to be exactly in the middle of 

 the incident pencil of rays. After reading off the corresponding 

 deflection of the multipliei", a vessel, of the form described in my 

 ' Physics' (5th ed., vol.i. p. 506), was placed before the open- 

 ing, and being filled, first with water and then with the liquids 

 mentioned before, in succession, the deflection caused in the mul- 

 tiplier was read off each time. 



It was observed, however, that when the circuit through the 

 thermo-battery was closed by the multiplier, a greater or less 

 deflection was effected, even when all radiation upon the thermo- 

 battery was prevented as much as possible. In addition, there- 



