0,96 CHEMICAL EFFECTS OF LIGHT. 



fhorter didances the furface coloured was larger, but in the 



fame time paler. At 22|: a green ring was formed, having its 



center without colour. 



At a medium When the guaiacura was expofed at diftances greater than 



focus no eflea ^3 or 23^, ihe furface coloured was alfo larger, but much 

 was produced ; * .. o ' 



paler than at equal diftances fhort of the focus; infomuch that 



at 24i, which correfponds with the principal focus of illumi- 

 nation, little or no efftft was produced in the fpace of one 

 minute, which was the time employed in other experiments. 

 It was manifeft therefore, that the chemical effedi of the mofl: 

 refrangible rays (which were now diverging beyond their focus) 

 was in this filuation counteracted by an oppolite adion equally 

 powerful, of the moil refrangible (not yet arrived at their fo- 

 cus;) and as it appeared probable that the power, which could 

 in one infiance prevent difcoloration, might alfo, when duly 

 applied, remove the fame colour after it had been produced. 

 At a longer fecal I, next expofed to the condenfed fpectrum, at various diftances 

 fiiftancc (namely from the lens, portions of the paper that had been previoufly 

 fran^^^ibie^ the' rendered uniformly green. A fecond focu* was now found at 

 original yellow the diftance nearly of 25|, in which the green colour was 

 was re ore . compleatly removed, and the guaiacum reftored to its pale yel- 

 low colour. 



It is unnecelTary to defcribe minutely the confequences re- 

 fuUing from variations of the diftance, as the effefls in this 

 inftaiue 'Aere neceftarily the counterpart to the preceding; 

 the circle of yellow was larger, when the paper was placed at 

 a greater diftance from the lens, and at the diftance of 25|^the 

 center remained green, furrounded by a yellow ring, corref- 

 ponding nearly (o the red and orange interior margin of the 

 annular priftmjic fpefitnun. 

 In carbonic acid 'T^^ flime experimenls being afterwards repeated in car- 

 gas the difoxid\- Iconic acid gas, only confirmed the opinion before entertained, 

 o'xid'acion was '^^^ ^''^ caufos lo.wliich the changes of colour were owing, but 

 pradi^able. afforded little additional information. In this gas the guaia- 

 cum could not be rendered green at any diftance from the lens, 

 but was fpeedily rcftored Irom green to yellow by expofu/e lo 

 the focus of red rays. 

 The removal of Since the removal of colour was obferved to take place in 

 ^"•T'l- r(J,t the lituation of tiie principal focus of heat, it feemed defirable 



oxiuanoii J- vva^ I I 



a!f. eflcfled by to afcerlain whether the prefence of light, or the circumftance 

 ^"J^^^'^P P'"'*''' of radiation had any influence in promoting this cffed. A 



piece 



