Mr. G. S. Cundell on the Gallo-nitrate of Silver. 101 



is divided into millimetres. The object of the notches is, that 

 a longer mark may be made at every five millimetres, and a 

 still longer one at every ten, in order to aid the eye in read- 

 ing. The waxed tube is now removed -to a leaden chest, 

 containing pounded fluor spar and sulphuric acid, slightly 

 heated, which etches it more successfully than a solution of 

 hydrofluoric acid. Previously, however, to being etched, it is 

 desirable to figure the number of millimetres at the space of 

 every ten ; and this is conveniently done by the steel pointer 

 G after being removed from E. 



We have thus an accurate measure of length etched upon 

 the tube, which should have been one of pretty uniform cali- 

 bre. The next point is to determine the true value of each 

 of the divisional marks. This is done by calibrating it through- 

 out all its length with small portions of mercury, say equal in 

 bulk to five grains of water. By this means the relative value 

 of each mark may be determined, and the proportion which 

 it bears to any given standard. The only possible error is 

 in the assumption that the tube is of even calibre between the 

 space occupied by the mercury ; but the quantity of this added 

 is so small that any such error becomes quite inappreciable. 



The convenience of this graduator is so great, that a long 

 tube may be beautifully divided in the course of a quarter or 

 half an hour. The standard tubes should be made of glass, 

 but the original divisions from which this standard is made 

 may be made on wood or any other material. 



XIX. On the Gatto-Nitrate of Silver of Mr. Fox Talbot, and 

 its Action upon Iodized Paper. By Geo. S. Cundell*. 



COME of the properties of this remarkable compound have 

 ^ been described by its inventor, and are sufficiently well 

 known ; the relations however of its constituent parts, and the 

 means by which the action of the compound may be regulated 

 (some knowledge of which is indispensable to the using it with 

 success), are less generally understood, and have not, so far 

 as I am aware, yet been described. 



When solution of the nitrate of silver is applied to iodized 

 paper, a surface is obtained which is extremely sensible to the 

 impressions of light: and, if gallic acid have been added to 

 the solution, the sensibility of the surface will be increased in 

 a remarkable manner. 



Preparations however of this kind are subject to a sponta- 

 neous and rapid change, independently of light; and, where 



* Communicated by the Author. 



