MOONLIGHT AND ARTIFICIAL FLAMES ARE INACTIVE. 



Besides which, there are two others, whose constitution is not well known ; one pre- 

 pared from an alcoholic solution of the double chloride of platinum and sodium, by 

 the action of chloride of potassium, and the other in a similar manner from the cyanide 

 of platinum. 



451. The changes which these bodies experience are of different kinds: some be- 

 come black and some turn white ; some, as the sulphate of nickel, undergo change of 

 crystalline arrangement. If we are to take the chloride of silver as a type of those 

 bodies which undergo partial reduction, it will be found probable that the change im- 

 pressed on them is only superficial, as analysis will show. But we cannot tell with 

 certainty whether a perfect reduction of some of these compounds takes place, or 

 whether it is a sub-salt of a dark-gray colour that results. By taking advantage of 

 the property which chloride of silver possesses of subsiding very slowly from neutral 

 solutions, so as to make them assume a milky consistency, we may present it in a state 

 extremely favourable to the action of the solar ray. For if a thick mass alone be ex- 

 posed, the central parts will not undergo the same change as the exterior, being shielded 

 by them from the sun. A milky solution will, after an exposure for a certain time, be- 

 come quite clear, the chloride precipitating, owing to the liquid becoming acidulous. 

 Mechanical agitation being then resorted to, to expose fresh surfaces of the precipitate 

 very frequently during a period of eight or ten days, and care being taken to suffer no 

 dust or other impurity to enter the vessel, it will be found that the powder has become 

 of a reddish gray, interspersed with little particles of unchanged white chloride ; these, 

 from their superior density, will have precipitated more readily than the gray particles; 

 washing and decantation will therefore readily effect a separation of them. One hun- 

 dred grains of the dark chloride thus treated yielded, on analysis, 79 - 3 of metallic silver ; 

 that quantity contains, therefore, 2O7 of chlorine ; it has lost by exposure 5'3 grains of 

 chlorine of the quantity originally contained in it 



452. Other analyses, of the same sample, furnished results not widely varying from 

 this, but such is not the case with analyses of different samples; these give sometimes 

 more, sometimes less chlorine ; they prove that the chloride of silver, as darkened by 

 light, is not a definite compound, but rather a mechanical mixture ; that the change of 

 composition is chiefly confined to the surface, and does not affect the interior of the 

 particles to any extent; it is true that microscopic observation show r s them to have 

 a uniform consistency and colour, but of course reveals nothing of their internal char- 

 acter. An error is frequently made by writers who describe the changes happening in 

 this partial reduction ; it is not, as they say, hydro-chloric acid which is evolved when 

 the chloride is under water, but it is chlorine, as is made very evident by the strong, 

 disagreeable odour of that gas when the experiment is conducted in close vessels. 



453. In addition to the list given above of substances changed by the chemical ravs, 

 there are some others which exhibit their action in a very marked manner. Chlorine 

 and hydrogen unite together with an explosion ; carbon and chlorine are also made 

 thus to unite in producing the per-chloride of carbon ; all kinds of vegetable colours 

 are bleached ; hydriodide of carbon and chloro-carbonic acid are always made by the 

 action of solar radiant matter. 



