Dr. Schunck on some Products derived from Indigo-blue. 293 



I. Let a?+px + q = be any cubic equation when deprived of 

 its second term ; then whatever be the signs of p and q, the fol- 

 lowing are sure and complete tests of the character of the roots : 



1. If (~£) '2T (%) , the roots are all real and unequal. 



2. If ( — ^j <( | J j two of the roots are imaginary. 



3. If ( — 5 j = (^ I , the roots are real, and two are equal. 



II. Let A 4 ^ 4 + A 3< 3? 3 + A 2 x 2 + A 1< r+N = be any equation of 

 the fourth degree, with its last term positive ; then all its roots 

 will be imaginary if the coefficients satisfy the condition 



(4A 4 A 2 -A^N>A 4 A,*; 



which condition becomes considerably simplified when A 3 =0, 

 and A 4 =l. 



September 4, 1865. 



XXXIX. On some Products derived from Indigo -blue. 

 By Edward Schunck, Ph.D., F.R.S.* 



MY experiments on the formation of indigo- blue, an account 

 of which I had the honour of presenting to this Society 

 several years ago, led me to make some inquiries regarding 

 the processes employed in tropical countries for the produc- 

 tion of indigo from the various plants yielding that dye- 

 stuff. I found that all the authors who have written on 

 the subject agree in affirming that the process of fermen- 

 tation, which is the one usually adopted for the purpose of 

 extracting the colour from the plant, requires to be con- 

 ducted with the greatest care, in order to yield a successful 

 result. Unless certain precautions are adopted, a product 

 of very inferior quality will be obtained ; in some cases, in- 

 deed, the colouring-matter is entirely lost. This will not 

 be surprising to any one who considers that though indigo- 

 blue, when once formed, is a very stable compound, the sub- 

 stance existing in the cells of the plant from which it ori- 

 ginates, and which I have named indican, is decomposed 

 with the greatest facility in various ways, that indigo-blue 

 is only one of its products of decomposition, and may be 

 formed or not, according to the nature of the process to 

 which it is submitted. With this sufficiently obvious expla- 



* From the Memoirs of the Literary and Philosophical Society of Man- 

 chester, S. 3. vol. iii. Session 1864-65. 



