Archceologia. 217 



cadmium it dries so soon that, in hot weather, the picture can 

 scarcely" be developed quickly enough. 



It has long been known that a solution of gold is decom- 

 posed by sunlight ; ivory may be gilt by washing with such a 

 solution, and exposure. When a neutral oxalate is heated with a 

 neutral solution of gold, metallic gold is precipitated. Light 

 produces the same effect as heat, and what at first is only 

 a stain will go on darkening for weeks, even if light is 

 excluded, especially in a damp atmosphere. This slow reduc- 

 tion which, after exposure to light, continues in the dark as 

 long as any unreduced gold remains, is common to all the salts 

 of that metal. The presence of chlorine, iodine, or bromine, 

 does not appear to render the salts of gold more sensitive ; 

 but if copper, iron, lead, tin, zinc, etc., are polished and ex- 

 posed to the action of chlorine, iodine, or bromine, in a dilute 

 state, there results a weak combination which is decomposed by 

 light, the metal being reduced. Several of the salts of mer- 

 cury are more or less affected by light. Bichromate of potash 

 is decomposed by it, when in contact with organic matter. Nor 

 is the action of light confined to compounds of the metals ; un- 

 der its influence, phosphorus dissolved in hydrochloric acid is 

 abandoned by the acid ; hence all salts which are soluble in 

 phospo-hydrochloric acid are sensitive to light when thus dis- 

 solved, though not so in other circumstances. If paper wetted 

 with a solution of acetate of copper in phospo-hydrochloric acid 

 is exposed under a negative, the parts acted on by light will 

 become of a grey colour, the acetate having become binoxide ; 

 this, if the paper is exposed to sulphuretted hydrogen, will be 

 changed to bisulphuret ; the picture will very soon vanish, how- 

 ever, by the bisulphuret forming sulphate, unless the copper is 

 changed for another metal, which may be effected by washing 

 the paper in water, and then plunging it into a solution of ni- 

 trate of bismuth, to which a small quantity of acetic acid has 

 been added. 



AKCELEOLOGIA. 



DISCOVERIES AT BATH— EXPLORATIONS AT RICHBOROUGH. 



It might be supposed that on the occasion of the British Associa- 

 tion's visit to a city so celebrated in the Roman period as Bath, 

 eager inquiries would be made after its antiquities, yet the remains 

 of Aqua3 Solis, as this place was called by the Romans, which are 

 now visible, are very few indeed, and the inquirer cannot fail to feel 

 disappointed. In one class, it is true, the museum of the Literary 

 Institution is rich ; those are, inscribed monuments of stone, such as 

 altars, sepulchral stones, etc. But in the class of miscellaneous 



