MEMORANDA. 245 



electrotype process, which, for many reasons, is to be pre- 

 ferred, as it does not distort the object, and is so minutely 

 faithful, that even the mere trace of organic matter left by a 

 slight finger-mark is perfectly copied. The method that I 

 have adopted is this — procure a small plate of metal highly 

 polished (a piece of daguerreotype plate answers extremely 

 well), and after gently heating it, rub a piece of bees'- wax 

 over the surface ; while this is still melted, wipe it neai-ly all 

 off again with a piece of rag, so as to allow a very thin film 

 to remain. When the plate is cold, arrange the DiatoviacecB 

 or other objects, previously moistened, upon the waxed 

 surface, heat the plate again to at least 212°, in order to 

 cement the objects on to it. The wax serves a twofold 

 purpose — first its interposition prevents the possibility of a 

 chemical union of the metallic deposit with the plate ; and 

 secondly, the object is securely held thereto by its agency. 

 The objects are now ready to receive a coating of copper. 

 If the battery is in good working order, three or four hours 

 will give a film sufficiently strong to bear removal ; when this 

 is stripped off, if the process has been properly managed, the 

 objects will be seen embedded in its surface. Whether they 

 are siliceous or organic they may be entirely dissolved out, 

 by boiling the cast in a test-tube, with a strong solution of 

 caustic potash, and afterwards washing with distilled water ; 

 the copper film may then be mounted in Canada balsam. 



By these means I have obtained distinct impressions of the 

 markings of some of the more difficult Diatomacece^ such as N. 

 Balticmn, P. Hipiwcampxis, &c., leaving no doubt of their promi- 

 nent nature. Care must be taken not to leave too much wax on 

 the plate, or either a clean deposit will not be obtained, or the 

 objects will be obscured by it. On the other hand, if too little 

 is left, the copper will insinuate itself underneath the structure, 

 and raise it from its place. Upon one occasion,! dried a section 

 of wood on to a metal plate by heating it. In this state it 

 appeared to be firmly adherent by its own resinous exudation. 

 On placing it in connection with the battery over night, in 

 the morriing I found the bare section on the outside of the 

 metallic deposit, upon which it had left a slight, though by 

 no means a good impression. Even when a thin film of a 

 non-conducting substance intervenes, the tendency of the 

 deposit is to get as near as possible to the conducting plate, 

 and in its endeavours to do so, it will fill every cavity and 

 pore, however minute. 



There is another method of obtaining metallic casts of minute 

 objects that gives some curious results, and is, therefore, worthy 

 of mention : it is done by stamping, or the same process in 



