400 Progress of Invention. 



is now made of iron, and yet it is the most perishable of the ordi- 

 nary metals. This circumstance becomes a source of serious anxiety 

 when we reflect that the vast majority of our railway bridges are 

 made of it. They cannot, as usually constructed, last more than a 

 very few years ; since, in a short time, the thin plates of which they 

 chiefly consist, will become so corroded as to be no longer capable 

 of supporting the weights they are required to bear. Various 

 modes of preserving the iron have been tried, but unfortunately 

 none of them have proved very effective ; oxidation goes on under 

 the very coating which is intended to exclude the atmosphere. 

 There is, however, now reason to hope that ingenuity, which has 

 vanquished so many difficulties, will conquer this also ; and one 

 step seems already to have been made towards the attainment of 

 the desired object. It has been found that the combination of phos- 

 phorus with the surface of the iron, at a high temperature, completely 

 preserves it ; since the coating of phosphate of iron thus produced 

 is a very stable compound, and one that is not acted upon by those 

 agents which usually produce so mischievous an effect on iron. 

 Should this mode of protection be found, in practice, and especially 

 on the large scale, easy of application, economical, and effective, 

 one of the most perishable may become one of the most lasting of 

 metals, a circumstance that would add enormously to its value. 



New Application of Chloride of Silver to Photography. — The 

 deterioration of photographs has been the source of great disappoint- 

 ment or anxiety. Various means have been used for preventing it, but 

 few of them have inspired much confidence, and it is yet uncertain 

 if any of them will prove thoroughly effective. A recent application 

 of chloride of silver, however, by Mr. G. Wharton Simpson, seems 

 to exclude those conditions from which a change in photographs 

 may be supposed to arise. He found that, if nitrate of silver is dis- 

 solved in collodion, and is changed into chloride by the addition of 

 a few drops of a solution of chloride of sodium, the resulting chloride 

 of silver is not precipitated, as might be expected, but remains, at 

 least for a considerable time, suspended ; so that although, by trans- 

 mitted light, the collodion is perfectly transparent, by reflected light 

 it is white like, opal glass ; and that, provided sufficient nitrate has 

 been used, it gives a vigorous image on paper. It is extremely 

 sensitive, and affords very good pictures wjth indifferent negatives, 

 while even the Wothlytype requires that they should have consider- 

 able density. The collodio-chloride paper has many advantages 

 over the albumenized. It is impossible that the latter can be of a 

 uniform quality, since every succeeding sheet immersed in the solu- 

 tion of sensitive salts exhausts it, and diminishes its efficiency with 

 regard to the next sheet. On the contrary, paper coated with the 

 collodio-chloride is, to the very last sheet, of exactly the same 

 strength. Perfect uniformity of effect is, therefore, easily attainable 

 with it. But the most important circumstance connected with this 

 preparation is the fact that, no soluble compound of silver being- 

 present, if the picture is properly fixed and washed, there is reason 

 to believe that it will be permanent. 



