138 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



ing specific ray or rays. The union of chlorine and hydro- 

 gen goes on under the influence of every ray of the spectrum, 

 but with greatest rapidity in the violet. The efiects of light 

 on chlorophyl show that the vegetable colors are destroyed 

 by rays complementary to those that have produced them. 



The second of the above propositions is supported by the 

 observations on tlie decomposition of the silver iodide, in re- 

 lation to which Dr. Draper develops a fact of much interest 

 to photographers, i. 6., that the ordinary collodion film ab- 

 sorbs only about one fourth of the whole actinic effect of the 

 rays falling upon it; the rest passes through and is lost. 

 Could the film be made to absorb the whole, its sensitiveness 

 would be correspondingly increased. 



The second j^roposition is especially supported by the di- 

 rect experiments with chlorine and hydrogen. The solar 

 rays having passed through a layer of chlorine, are unable to 

 cause the combination of a mixture of hydrogen and chlorine 

 on which they are allowed to fall ; without the intercepting 

 layer of clilorine, the solar rays cause the immediate combi- 

 nation of tlie two gases. Further experiments with absorb- 

 ino' media show that the more i-efrani2jible ravs arc the ones 

 effective in causing the union of chlorine and hydrogen, and 

 that, furthermore, the rays that are specially eff*ective are 

 those corresponding to the bands common to the spectra of 

 the two gases. The process of union begins after the lapse 

 of a certain time, during which the rays entering the mixture 

 have been acting upon it to j^repare it for the subsequent un- 

 ion. The actual union is a progressive phenomenon, the 

 rapidity of which increases with the intensity and quantity 

 of light. 



The action of light on the chlorine compounds of silver is 

 precisely similar to that on the compounds with hydrogen ; 

 and " there is to practical photographers an advantage, both 

 as respects time and correctness in light and shade gained 

 by submitting a sensitive surface to a brief exj30sure in a dim 

 light, so as to pass it through its preliminary stage." 



PROTECTION OF SHIPS' COMPASSES. 



The possibility of protecting ships' compasses from the dis- 

 turbino; eff'ects of the varying^ action of the iron of the vessel 

 has led several eminent electricians to devise means to secure 



