VOL. LXI.] PHILOSOPHICAL TRANSACTIONSi 107 



Washing it with water. It is to be dried slowly, for otherwise it is apt to 

 eatch fire. 



Then follows an account of a method of dying wool and silk, of a yellow 

 colour, with indigo ; and also with several other blue and red colouring sub- 

 stances. 



The Saxon blues have been known for some time ; and are made by dissolving 

 indigo in oil of vitriol, by which means the indigo becomes of a much more 

 lively colour, and is extended to such a degree, that it will go very far in dying. 



A receipt for making the best Saxon blue will, Mr. W. doubts not, be agree- 

 able to many; he therefore gives the following, which produces a very fine colour, 

 and never fails of success. Mix 5 1 of the best powdered indigo, with § 4 of 

 oil of vitriol, in a glass body or matrass: and digest it- for one hour with the heat 

 of boiling water, shaking the mixture at difitrent times; then add § 12 of water 

 to it, and stir the whole well, and when cold, filter it. This produces a very rich 

 deep colour; if a paler blue be required, it may be obtained by the addition of 

 more water. The heat of boiling water is sufficient for this operation, and can 

 never spoil the colour; whereas a sand heat, which is commonly used for this 

 purpose, is often found to damage the colour, from its uncertain heat. 



Indigo, which has been digested with a large quantity of spirit of wine, and 

 then dried, will produce a finer colour than the former, if treated in the same 

 manner, with oil of vitriol. No one, that he knew of, had before made use of the 

 acid of nitre, instead of the acid of vitriol; and it is by means of the former that 

 the yellow colour is obtained: it was nevertheless natural to use it, on account of 

 its known property of making yellow spots, when dropped on any coloured cloth. 

 The acid of salt does not dissolve indigo, and therefore is of no use in dying. 

 Mr. W. further communicates a receipt for making the yellow dye. 

 Take ^ -^ of powdered indigo, and mix it in a high glass vessel, with % 2 of 

 strong spirit of nitre, previously diluted with 5 8 of water; let the mixture stand 

 for a week, and then digest it in a sand heat for an hour or more, and add 5 4 

 more of water to it; filter the solution, which will be of fine yellow colour. 

 Strong spirit of nitre is liable to set fire to indigo; and it is on that account that 

 it was diluted with water, as well as to hinder its frothing up. 5 2 ^ of strong 

 spirit of nitre will set fire to 5 -i- of indigo ; but, if it be highly concentrated, a 

 less quantity will suffice. If the indigo be digested 24 hours after the spirit of 

 nitre is poured on it, it will froth and boil over; but after standing a week or less, 

 it has not that property. 



One part of the solution of indigo in the acid of nitre, mixed with 4 or 5 

 parts of water, will dye silk or cloth of the palest yellow colour, or of any shade 

 to the deepest, and that by letting them boil more or less in the colour. The 

 addition of alum is useful, as it makes the colour more lasting; according as the^ 



e 2 



