CONTENTS. 



necessary. The seat or support. The background. Appropriate 

 dresses. Ladies' 1 dresses. Arrangement of the shadow. Reflecting 

 camera Page 215 



MEMOIR XYI. 



ON THE CHEMICAL CONDITION OF A DAGUERREOTYPE SURFACE. 



Mercury exists all over a daguerreotype surface. There is no superpo- 

 sition of the parts. The shadows have metallic mercury; the lights 

 silver amalgam. No iodine is ever evolved from the plate. Action of 

 a solution of gum and one of gelatine in tearing off the films. The 

 starch experiments. The etching of daguerreotypes 222 



MEMOIR XVII. 



ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE CHEMICAL 

 RAYS AND THOSE OF RADIANT HEAT. 



The chemical rays are absorbed. Photographic effects are transient. 

 The chemical rays are not conducted ; they become latent. Optical 

 qualities control chemical action. The active rays are absorbed and 

 the complementary reflected. Relation of optical conditions and chem- 

 ical affinities 230 



MEMOIR XVIII. 



DESCRIPTION OF THE CHLOR-HYDROGEN PHOTOMETER. 



Properties of a mixture of chlorine and hydrogen. It is acted upon by 

 lamplight, an electric spark at a distance, etc. The gases unite in pro- 

 portion to the amount of light. Mode of measuring out known quan- 

 tities of radiations. The maximum action is in the indigo space. 

 Construction of the instrument. The gases are evolved by electricity 

 and combined by light. Theoretical conditions of equilibrium. Pre- 

 liminary adjustment. Method of continuous observation. Method of 

 interrupted observation. Remarkable contraction and expansion . . 245 



MEMOIR XIX. 



ON MODIFIED CHLORINE. 



Description of the experiment. The change in the chlorine is not tran- 

 sient. There are two stages in the phenomenon. Rays are absorbed 

 in producing this change. The indigo ray is absorbed. The action is 

 positive from end to end of the spectrum. The indigo ray forms hy- 

 drochloric acid and also produces the preliminary modification. 



