96 Dr. A. Waller's Observations on certain 



on glass will cause the formation of a deposit. Wood, glass, 

 slate, and other similar substances, all have equal power in 

 this respect, but metallic substances are less active. Other 

 polished surfaces may be used instead of the glass plate, and 

 I have formed these images on quartz and agate with the same 

 effect. 



The difference of crystalline texture exerts no influence, 

 but the images seem to be with more difficulty produced on 

 polished silver and copper than on a vitreous surface. 



A very slight degree of friction will excite the formation of 

 an image, although a moderate degree of pressure is more fa- 

 vourable. 



Electricity exerts no influence in the formation of these 

 images. In one experiment, in order to diminish the friction, 

 I adapted two fine wires of a spiral form to a battery suffi- 

 ciently strong to decompose water freely. These wires were 

 moved through the solution in. various directions, and the 

 marks of the passage of the two poles became equally appa- 

 rent without any difference on either side ; and when after- 

 wards disconnected from the battery and used in a similar 

 manner, they produced the same effects. 



It is remarkable with what fidelity the traces of lines be- 

 come visible in this manner. Letters thus formed by a pen, 

 are much more faithfully rendered than when written on paper 

 with ink, and lines may be formed which are scarcely visible 

 to the naked eye. Microscopic inspection shows this extreme 

 exactness to a much greater degree than could have been an- 

 ticipated; for we see a simple line become as it were decom- 

 posed into a number of parallel lines, which represent the 

 point of contact between the two solids (see Plate III. fig. 2). 

 These lines are composed of very minute and confused cry- 

 stals, of an irregular appearance and joined together. Their 

 diameter varies from 0*02 of a millimetre to about double that 

 size. Between these parallel lines are frequently seen others 

 still more minute. The other crystals which become deposited 

 by the common crystalline powers over the untouched parts 

 of the glass, are much larger than either of these. When the 

 point of intersection of two lines is examined under the micro- 

 scope, we perceive the appearance represented. While cry- 

 stalline masses are in process of formation, it is impossible to 

 prevent the deposition of crystals on other parts of the glass; 

 but if while these are fresh they are subjected to a sharp cur- 

 rent of water, the irregular crystals are mostly carried away, 

 while the images are left almost intact. It is therefore evident 

 that the same power which causes this deposit, renders them 

 more adherent to the surface of the glass than the other cry- 



