292 Dr. E. Budde on the Action of Light 



were closed by simply sealing them up, subsequently by means 

 of soldered-up glass stopcocks. Concentrated sulphuric acid 

 previously saturated with chlorine served as index-fluid. The 

 viscosity of this liquid renders the thermometer rather unsensi- 

 tive ; yet I preferred it to any other on account of its stability 

 and its small vapour-tension. Only in a few confirmatory expe- 

 riments I used chloride of carbon, CC1 4 , as indicator, in order to 

 show that the phenomenon observed was not caused by the action 

 of the sulphuric acid, and also to form an idea on the duration 

 of the reaction [vide infra). The bulbs were so placed that either 

 of them could be exposed to any portion of the solar spectrum ; 

 their distance from the prism varied from 1 to 2 metres ; and 

 their shadows, according to their position, covered from one third 

 to one sixth of the visible part of the spectrum. The index was 

 illuminated with gas-light, and its position observed with respect 

 to one of the cross wires of a telescope. 



Equilibrium of temperature having been established, the cross 

 wire was made to coincide with the end of the index, the light 

 then made to act, and the variation in the position of the index 

 observed and estimated. Let us designate one of the bulbs by 

 A, the other by B, and call positive any motion of the index 

 from A towards B ; then the results of one of the series of ob- 

 servations may be stated as follows : — 



Illumination of 



A 



Displacement 



r^ 



A 



A. 



B. 



of index. 



Ultra-red + red. 



Dark. 



+ | to 1 millim. 



Red -f yellow. 



>) 



-f J millim. 



Blue + violet. 



)) 



5 to 6 millims. 



Trace of blue + violet 1 

 + ultra-violet. J 



Dark. 



6 to 7 „ 



Ultra-violet. 



a 



4 



Dark. 



Red. 



— ^ or less. 



„ Violet -f adj oinin g rays 



. —5 millims. 



Red and yellow. Ultra-violet, limit. 



-f- not measurable. 



The whole of the red"! 

 end of the spectrum. J 



Violet end of the 

 spectrum. 



>— 2 to 3 millims. 



Dark. 



Dark. 



+ 5 millim.* 



The experiments were repeated several times with substan- 

 tially the same results ; only it occasionally occurred that the 

 sulphuric acid would not move at all ; but in such cases it would 

 no more obey a slight increase of temperature ; and when the 



* Instead of ; as the series of observations extended over about twenty 

 minutes, this small difference is easily accounted for by a slight difference 

 in temperature of the bulbs. 



