Progress of Invention. 



silver, has long attracted the notice of chemists and photographers ; 

 but the reason of this difference, though involving very important 

 considerations, has not been hitherto understood. Even unex- 

 plained facts are valuable, but an explanation very often renders 

 them infinitely more so. Such is the case with regard to the pecu- 

 liarity of iodide of silver. Dr. Vogel, of Berlin, has recently thrown 

 a new light on the conditions which render this salt sensitive, or 

 the contrary. It was long known to photographers that iodide of 

 silver, formed with excess of iodide of potassium, is totally insensi- 

 tive to light ; but that, when the nitrate has been left in excess, it 

 is highly sensitive. In looking for the reason of these facts, it 

 occurred to him that certain compounds are decomposed immedi- 

 ately by heat, while others, in addition, require the presence of a 

 body that has an affinity for an element which the heat will set free. 

 Thus, heat by itself will decompose oxide of silver, but it will not 

 decompose oxide of iron, unless hydrogen is present to unite with 

 the liberated oxygen. The consideration of this difference in the 

 powers of heat led him to suspect that the action of light might be 

 analogous ; that, by itself, it might decompose some compounds, 

 while it would have no effect upon others, except in presence of a 

 body which could unite with an element it should disengage ; and 

 further inquiry showed that he was right. Here then was found 

 an explanation of the apparent anomalies connected with the salts 

 of silver. The chloride, for instance, is decomposed by light alone ; 

 the iodide requires the presence of some substance that will com- 

 bine with the liberated iodine : such a substance is found in nitrate 

 of silver, and in tannin, bodies which, at first sight, would seem to 

 have nothing in common. The principle once discovered, it was 

 found to be susceptible of very extensive application ; any sub- 

 stance ready to combine with the liberated iodine will answer in- 

 stead of the nitrate of silver, or the tannin. It is chiefly the excess 

 of nitrate, present in wet plates, that renders them more sensitive 

 than those which are dry ; but such a substitute for nitrate of 

 silver may be found as will impart an equal sensitiveness to dry 

 plates — one of the greatest boons that could be conferred on the 

 photographer. 



Enamelled Silver. — The art of enamelling is very ancient • its 

 products are both very beautiful and very durable. Its utility has, 

 however, been circumscribed by various circumstances, among which 

 is to be reckoned a restriction as to the material on which the enamel 

 is produced. This has usually been copper or gold : the former is 

 objectionable for several reasons, the latter from its dearness. 

 Silver would be a very appropriate material ; but, until recently, it 

 was not possible to use it in enamelling. M. Auguste Geffroy, of 

 Paris, has, however, after a long series of experiments, discovered 

 the means of employing it for the purpose, and intends very shortly 

 to make known the causes which have led to the failure of others, 

 and to his own success. 



Improvement of the Air-pump. — Certain imperfections are' 

 observable in the ordinary air-pump which, hitherto, it has been 



