Mr. W. B. Croft on Breath Figures. 181 



clean and dry with a leather : the coins may be used just as 

 they usually are, or chemically cleansed, it makes no differ- 

 ence. The tin-foil and the upper coin are connected to the 

 poles of a Wimshurst machine which gives 3 or 4 inch sparks. 

 The handle is turned for two minutes, during which one-inch 

 sparks must be kept passing at the poles of the machine. On 

 taking up the glass one can detect no change with the eye or 

 the microscope ; but when either side is breathed upon, a clear 

 frosted picture appears of that side of the coin which had faced 

 it : even a sculptor's mark beneath the head may be read. 

 For convenience those parts where the breath seems to adhere 

 will be called white, the other parts black. In this experi- 

 ment the more projecting parts of the coin have a black 

 counterpart, but there is a fine gradation of shade to corre- 

 spond with the depth of cutting in the device : the soft 

 undulations of the head and neck are delicately reproduced. 



The microscope shows that moisture is really deposited over 

 the whole surface, the size of the minute water granulation 

 increasing as the point of the picture is darker in shade. 



There seems to be no change produced by the use of coins 

 of different metals. 



If sparking is allowed across the glass instead of at the 

 poles of the machine, traces of metal are sometimes deposited 

 beyond the disk of the coin, but not within it. 



Around the disk is a black ring J inch broad : sometimes 

 the milling of the coin causes radial lines across this halo. 



If carefully protected there appears to be no limit to the 

 permanence of the figures, but commonly they are gradually 

 obscured by the dust gathered up after being often breathed 

 upon : some of the early ones, done more than two years back, 

 are still clear and well defined in the detail. 



It is possible to efface them with some difficulty by rubbing 

 with a leather whilst the glass is moist. They are best pre- 

 served by laying several together when dry and wrapping 

 them in paper : they are not blurred by this contact. 



It is a curious fact that certain developments take place 

 after a lapse of some weeks or months. The dark ring 

 around the disk gradually changes into a series of three or 

 four, black and white alternately ; other instances of such a 

 change will be noted below. 



Let it be noticed that in coin pictures the object is near to, 

 but not in contact with, the glass : for in the best specimens 

 the rim of the coin keeps the inner part clear of the surface. 



Obviously a small condenser is made by the coins : it is 

 not essential ; at the same time images made by a single coin, 

 put to a single pole, are inferior. 



