272 On tlie limine Spencer. 



vanning in colour from the original blue. The changes wcrei 

 to a greenish blue, a green, an olive green, a yellowish 

 green, a yellow, and so on to a buft" colour, where I ceased 

 from further prosecuting the search. 



I then tried the same experiment with a cake of New- 

 man's Prussian blue; and, rcpeatin;^ the process quicker, I 

 obtained from the same orio^inal quantity of colour first laid 

 on the knite no less than eighty-5i>: d 'terent tints, each va- 

 rying in colour as in the former experiment. 



Indigo treated in a similar manner did not change its tint 

 in the kast degree under the process. 



This discovery may lead to very important consc ]uences 

 in the theory of colour-making ; and it fiu-nishcs some cu- 

 rious, facts, namely, that the Chinese are acquainted with 

 the art of making a colour similar to the I'russian blue ; and 

 that indigo shovild be preferred to the Prussian blue when 

 xve \\ ish our colours to be durable, and not suljject to change 

 on coming in contact with iron .- it should also serve as a 

 caution never to employ the pallet knife or iron in any 

 form in treating Prussian blue. 



It would appear from the result of this experiment, either 

 that the Prussie acid, one of the constituent principles of 

 Prussian blue, is not saturated in that combination with 

 iron, but is still able to exercise an action on that metal 

 in its metallic state, or else that the acid itself becomes 

 decomposed, giving up its oxygen to the iron, ami pro- 

 ducing the ochrcy tint which changes gradually the blue to 

 green. 



We are possessed of orte tinge only of green colour pro- 

 duced from iron, namely, Prussian green, with the process 

 for obtaining which I am unacquainted. 



Allow me before concluding to mention another fact 

 with which some of your readers may not be acquainted. 

 The superiority of the Chinese colours over those of Europe 

 is owing chiefly to their being ground much liner, and be- 

 ing mixed with size instead of gum, wliicu prevents their 

 having any gloss. 



XLIX, On the Marine Spencer, inrentni /// Knight 



Si'KXCKRj llsCJ. 



Jlx our last~ Number W? gaVC a! short description of this in- 

 vention. To show its great utility, we need only to mention 

 a few' of the many possible cases in which it may be em- 

 ployed 



