from the Dissociation of Water Vapour. 157 



we reflect, however, on the supposition that emulsions con- 

 taining silver salts are capable of responding and giving a 

 permanent record of all waves of light, even in the portion of 

 the spectrum considered most actinic, when the waves exceed 

 a certain intensity, we are conscious that we rely without 

 proof upon an infinite range of photochemical action ; and 

 indeed I show in this paper the existence of a selective rever- 

 sibilitv produced on the photographic plate by powerful 

 discharges producing light of great intensity. 



Realizing the importance of studying the behaviour of 

 o-ases under different forms of excitation, I have collected in 

 the rooms devoted to spectrum analysis in this laboratory 

 three forms of apparatus : an induction-coil actuated by a 

 very efficient liquid break, giving a spark of 30 inches in air ; 

 a step-up transformer, excited by an alternating current, pro- 

 ducing with glass condensers of about '3 microfarad dis- 

 charges of an inch in length of great body ; and a storage- 

 battery of twenty thousand cells. A plant of this nature I 

 conceive to be necessary in the present stage of spectrum 

 analvsis : for molecular motions excited in rarefied gases vary 

 oreatlv with the kind of electrical discharge. In the appli- 

 cation of photography to spectrum analysis one is immediately 

 confronted with the necessity of submitting the gas to a com- 

 paratively long electrical stimulus in order to obtain a negative. 

 Even with a concave grating of short focus several discharges 

 are necessarv with a narrow slit. Each discharge is capable 

 of modifying the condition of the gas. This fact is well 

 recognized by taking successive photographs upon the same 

 plate with different strengths of current. A simple form of 

 plate-holder enables this to be clone. One obtains a striking 

 example of the instability of a spectrum-tube filled, apparently, 

 with dry hydrogen when one subjects it first to very powerful 

 discharges from a glass condenser of '6 microfarad charged 

 by a storage-battery of twenty thousand cells, with practically 

 no self-induction in the circuit, and follows this excitation by 

 an alternating current of much less quantity. The powerful 

 discharge gives what I term the spectrum arising from the 

 dissociation of water-vapour ; and the alternating current 

 gives the spectra of argon. This results, J suppose, from 

 the oxidization of traces of air in the tube under the action 

 of the dissociation of the water-vapour. The presence of 

 hydrogen is concealed. On cooling, the tube again shows 

 the four-line spectrum of hydrogen. The period of the con- 

 denser-discharges which I have employed varied from one 

 five-hundred thousandth of a second to one millionth. The 

 practically instantaneous current, therefore, varied from five 



