130 SCIENTIFIC AMUSEMENTS. 



solution and weak caustic soda be used as developer, yellow 

 characters will appear ; and the same result will be obtained by 

 writing with acetate of lead and developing with iodide of 

 potassium, or better, with a moderately weak solution of yellow 

 chromate of potassium. 



In all these instances (and many other similar ones which will 

 occur to you as you become more acquainted with the properties 

 of substances and their chemical nature) the essential action going 

 on in rendering the characters legible is the formation of a pre- 

 cipitate of some kind or other; but there are other kinds of 

 " sympathetic inks," which act on a different principle. For 

 example, writing done with a weak solution of nitrate of cobalt (a 

 pink fluid) will be all but invisible when dry, but on heating the 

 paper before the fire the characters show forth in blue, gradually 

 fading away on removal from the fire and exposure to air (especially 

 if damp) and reappearing on warming again. This is due to a 

 different kind of chemical action brought about by the heat, and 

 essentially consisting in the expulsion of water contained in the 

 pink cobalt compound, whereby a much more deeply-tinted blue 

 substance is formed. On exposure to cool moist air this blue 

 compound gradually absorbs moisture again, and becomes recon- 

 verted into the lighter pink compound, so that the writing fades, 

 and becomes almost invisible. 



Expt. 130. Summer and Winter Pictures. A pretty applica- 

 tion of the above principle is the " Changeable Landscape " some- 

 times sold, which in ordinary weather represents an outdoor scene 

 of some kind with a winter aspect. On carefully warming the 

 picture before the fire, the grass becomes green and the trees 

 clothed with verdure instead of snow, and the whole scene changes 

 from winter to summer. On leaving the picture to itself in a 

 cool place, the colour will fade away, and the old winter view 

 will be restored. This effect is brought about by carefully brushing 

 over the parts of the picture to be treated with solutions of com- 

 pounds of cobalt mixed with other substances, capable of modifying 

 the colour of the more highly-tinted products formed on heating 

 and driving off moisture, the strength and nature of the liquid 

 regulating the tint produced on warming. Copper chloride added 

 to cobalt chloride solution gives a greenish or yellowish hue 

 according to the quantity added ; nickel chloride and perchloride 

 of iron also give green shades when in the proper proportion, 

 whilst zinc salts develop a red tone. Overheating must be 

 carefully avoided, otherwise the metallic compounds will be so 

 acted upon as to produce brown stains on the paper, which will 

 not fade away on standing in a cool place. 



