Change ol served in the Colour of Prussian Blue. 271 



line humour is not of an uniform density throughout ; it 

 is therefore essential that attention be given to preserve 

 that humour entire for this operation. I found the weio^ht 

 of a very fresh crystalhnc of an ox to be 30 grains ; and its 

 specific gravity v/as, as I before stated, 10765. I then pared 

 away all the external part, in every direction, till there re- 

 mained but 6 grains of the centre ; and the specific gravity 

 of these 6 grains I found to be 1 1940. From this it would 

 seem that the density increases gradually from the circum- 

 ference to the centre. 



It is not surprising that the crystalline humour should be 

 subject to disorders, it being wholly composed of animal 

 matter of the most perishable kind. Fourcroy says that it 

 is sometimes found osseous in advanced age. Albumen ii 

 coagulated by many methods; and, if we suppose that the 

 same changes can take place in the living eye as in the dead 

 animal matter of the chemists, it will be easy to account 

 for the formation of the cataract ; a disorder which cannot 

 be cured but by the removal of the opake lens, If a suffi- 

 cient number of observations were made respecting the fre- 

 quency of the cataract in gouty habits, some important con- 

 clusions might be drawn as to the iniiucnce of phosphoric 

 acid in causing the disorder, by the common eflect of 

 acids in coagulating albumen. 



XLVIII. On a Change ohscrved In the Colour of Prussian 

 Blue I'lj coming in Contact with Iron. By Thomas 

 Gill, Esq. 



To Mr. Tilloch. 



H' 

 AviNG frequently discoursed with you on the various 



colours producible Iroin iron, I make no apology for trou- 

 bling you with this letter. Happening this morning to be 

 grmding some Chinese blue colour with parchment, size, 

 and water, I noticed that a knife I employed in mixing it 

 became of a greenish tmge : upon this I resolved to prose- 

 cute the experiment, when the following singular result was 

 obtained : — After spreading a little of the colour upon the 

 blade of the knii'e with a camel's hair pencil, I diluted and 

 took off enough of the colour to form a tint upon paper, 

 and as soon as that was laid on proceeded to take oli a se- 

 cond, a third, &c. portion, without adding more colour to 

 the knife, until I had obtained thirty-six diiferent tints, each 



varying 



