THE INSTRUMENT CONSISTS OF A MIXTURE OF CHLORINE AND HYDROGEN. 187 



zing process, as is often the case, the small quantity of vapour it absorbs will destroy 

 every picture, or, at all events, increase the time required in the camera enormously. 

 The reason of this is easily understood. Suppose a plate in such a frame be placed in 

 the camera, or, what comes to the same thing, suppose a particle of iodine has fallen 

 into the camera, or that the wood has in any way absorbed an electro-negative vapour ; 

 as fast as the light makes its impression on the sensitive surface the vapour detithonizes 

 it, and unless the light is quite intense or the exposure much prolonged, a very feeble 

 proof, or no proof at all, will be obtained. In the same way, the difficulties are greatly 

 increased in the process of mercurialization ; for the temperature resorted to being high, 

 if there is the least particle of iodine about the box, the picture will be inevitably and 

 instantly detithonized and ruined. 



834. We ought, therefore, never to allow iodine, or bromine, or chlorine, to have ac- 

 cess to the apartment or the apparatus in which Daguerreotype operations are being 

 conducted. 



CHAPTER XVI. 



DESCRIPTION OF THE TITHONOMETER, AN INSTRUMENT FOR MEASURING THE CHEMICAL 



FORCE OF THE INDIGO-TITHONIC RAYS. 



(from the London, Edinburgh, and Dublin Philosophical Magazine for December, 1843.) 



CONTENTS : The Instrument consists of a Mixture of Chlorine and Hydrogen. It is 

 acted upon by Lamp Light, an Electric Spark at a Distance, $$c. Chlorine and Hydro- 

 gen unite in Proportion to the Amount of Light. Mode of measuring out known 

 Quantities of Rays. The Maximum of Action is in the Indigo Space. Construction 

 of the Instrument. Theoretical Conditions of Equilibrium. Preliminary Adjust- 

 ment. Method of continuous Observation. Method of interrupted Observation. 

 Remarkable Contraction and Expansion. 



835. I HAVE invented an instrument for measuring the chemical force of the tithouic 

 rays which are found at a maximum in the indigo space, and which from that point 

 gradually fade away to each end of the spectrum. The sensitiveness, speed of action, 

 and exactitude of this instrument, will bring it to rank, as a means of physical research, 

 with the thermo-multiplier of M. MELLONI. 



836. The means which have hitherto been found available in optics for measuring 

 intensities of light, by a relative illumination of spaces or contrast of shadows, are ad- 

 mitted to be inexact. The great desideratum in that science is a photometer which 

 can mark down effects by movements over a graduated scale. With those optical 

 contrivances may be classed the methods hitherto adopted for determining the force of 

 the tithonic rays by stains on Daguerreotype plates, or the darkening of sensitive pa- 

 pers. As deductions drawn in this way depend on the opinion of the observer, they 

 can never be perfectly satisfactory, nor bear any comparison with thermometric results. 



