1 6 Mr. Warren De la Rue on the Stnicture 



quently the process is stopped. This exhaustion of the elec- 

 trolyte is the primary cause of the difficulties of the process. 



If, for example, an attempt be made to produce a solid 

 medal by depositing between the respective matrixes of the ob- 

 verse and reverse, the attempt would undoubtedly fail ; the 

 opposite and approaching deposits never join and form one 

 solid piece. The cause of this is sufficiently obvious, and can 

 be easily traced to the exhaustion of the electrolyte ; for as 

 the opposite deposits approach they render the cavity smaller 

 and smaller, and at last it becomes so reduced that its ca- 

 pillarity interferes with the renewal of the liquid, and eventu- 

 ally the action ceases, a cavity containing the exhausted liquid 

 remains, and no true juncture takes place. 



The following experiment proves, in a striking manner, that 

 the preceding is not merely a conjectural case. I prepared a 

 matrix by cutting angular lines about one-twentieth of an inch 

 deep in a metallic plate, the angle of the opening being about 

 35 degrees. On this was deposited copper to about the eighth 

 of an inch in thickness ; the deposit was what would be termed 

 exceedingly good, and, to a practised eye, uniform, there being 

 but a slight indication of the original lines at the back ol 

 the cast; yet the copper could be broken with great facility 

 through the centre of each line, in a plane bisecting the angle, 

 presenting to the eye a very smooth and uniform fracture, and 

 giving the idea of its having been rather purposely cut through 

 than broken at random. Here we have an example of the 

 interstitial space being not only left between the walls of the 

 original groove, but existing through the additional eighth of 

 an inch of deposited metal, from the obvious cause of the in- 

 capability of the included liquid acting the part of a cathode. 

 Plate I. fig. 1 is a diagram of the section through the matrix 

 and deposited copper ; A being the matrix, B the deposited 

 metal. 



A matrix from a wood engraving presents a good example 

 of the effect which the form of the mould has on the resulting 

 deposited counterpart : in it we have a vast assemblage of 

 minute grooves, the reverse of the lines in the engraving j 

 these grooves are never perfectly filled, from the cause before 

 stated, and the deposit is consequently cut up into an assem- 

 blage of minute pieces, joined apparently though not in reality, 

 and nicely held together by the intricacy of their interlacings. 

 The sky, from its consisting of a number of straight lines in 

 the engraving, is especially tender and troublesome to obtain 

 perfect. Though 1 have chosen the copy of a wood engra- 

 ving as an example, from its presenting an extreme case of 

 difficulty, yet the same effect is produced under other circum- 



