19. (Hewlet's.) Resembles Spencer's (No. 28).

20. (INSTANTANEOUS.) Moisten the hair first with a solution of nitrate of

silver in water (1 to 7 or 8), and then with a weak solution of

sulphydrate of ammonium. The colour of the hair, unaltered by the silver

solution, instantly turns black when moistened with the sulphuret. See EAU

D'AFRIQUE.