18 Mr. Warren De la Rue on the Structure 



linear; the section is a cutting made with sharp scissors 

 through a plate of about the fortieth of an inch in thickness, 

 having the character of the lesser crystalline, and appeared to " 

 the unassisted eye perfectly polished and compact. Fig. 3 is 

 a representation of the back of the same specimen, showing 

 clearly its crystalline structure ; this is likewise magnified 100 

 times. Even the most compact specimens present, under a 

 sufficient magnifying power (see fig. 5), the same porous ap- 

 pearance; the pores are smaller, it is true, but greater in 

 number, and the character of the deposit is the same. I here 

 again repeat that the electro-metallic deposit is essentially 

 crystalline in structure; in fact it is but a tissue of crystals 

 i?iterlactng but tiot ad/jerivg. We may diminish the power of 

 the battery with respect to the quantity of metallic salt present 

 in the electrolyte, so as to obtain, by these favourable circum- 

 stances, large and well-formed crystals, — we may go on in- 

 creasing the power, and produce the crystals more and more 

 hurriedly, and consequently smaller and smaller and less per- 

 fectly formed, but we ultimately reach a point in the quantity 

 of electricity transmitted, that, if we increase it, the electrolyte 

 cannot be renewed with sufficient rapidity at the surface of 

 the cathode, and we have larger spaces left unfilled, thus pro- 

 ducing the sandy deposit. Lastly, we may increase it to so 

 great an extent that the metal assumes the form of the spongy 

 deposit; but still all are crystalline. 



The ridges or lines which are frequently seen at the back 

 of electro-casts, placed vertically in the trough, more especially 

 in those where there are sharp angles jutting out from the 

 matrix, are produced by these points impeding the upward 

 flow of partially exhausted liquid, and causing it to run in 

 little detached streams ; where these exist t/iere the deposit is 

 formed less quickly than in those parts where the liquid is 

 stronger, consequently we have a hollow or groove produced, 

 which remains permanent. To produce an interchange of 

 the fluid in the precipitating troughs, a constant stream of 

 fresh liquid was caused to run in at the bottom whilst the 

 weak fluid overflowed at the top: the resulting current in 

 meeting the upward flow of the fluid at the surface of the elec- 

 tro-cast produced a series of vortices, and changed the vertical 

 ridges into a series of curiously-curved lines, which were 

 equally objectionable ; hence this scheme had to be rejected. 



Fig. 4 is a magnified representation of a curious electro- 

 deposit of silver, kindly furnished me by Mr. Napier, who has 

 named it "silver sponge," from its resemblance to that sub- 

 stance. Under the microscope it is a most splendid arborescent 

 assemblage of crystals, and presents us with an extreme case 



