BREATH-FIGURES. 31 9 



rendered visible, because the vapour contained in the 

 breath is deposited unequally upon the clean portions of 

 the glass and upon those still covered with condensed air. 

 As soon as the deposited vapour disappears, the writing 

 disappears also, but appears when the plate is again 

 .breathe'" upon, and this will even be the case for a day 

 ifter. In order to remove the layer of air uniformly, 

 [he plate must be strongly rubbed with a piece of cloth 

 ill over the surface ; the capability of the plate to re- 



FIG. 206 (| real size). 



>roduce the writing is by no means destroyed by 



nerely wiping the surface with the cloth. A simple 



node of producing such c breath figures ' is, to cut in 



tiff paper a figure with not too delicate outlines, for 



xample a star, as in fig. 206. The paper is then placed 



iipon a glass plate and breathed upon, so that the 



portions of the glass where the paper has been removed 



jre covered with vapour. When this vauour, after the 



;aper has been removed, is allowed to evaporate, so 



;bat nothing is seen of the figure, it will immediately 



sappear on breathing again upon the plate, although 



i this case the figure cannot be reproduced as many 



mes as in the former case. The explanation of this 



henomenon rests on the fact that the adhering layer of 



