SOLID SOLUTIONS. 131 



its plausibility, met with a ready acceptance in many 

 quarters. 



According to Witt the state of the dye-stuff in the fibre 

 is one of solid solution, and many analogies were advanced 

 in support of this assertion. For example, dyed materials 

 show the colour, not of the solid dye-stuff, but of the dye- 

 stuff in solution, when there is a difference of colour between 

 the two states. Solid fuchsine is green, its aqueous solutions 

 are red, and so also are materials dyed with it. The dye- 

 stuff rhodamine in the solid state exhibits no fluorescence, in 

 solution it does, and silk dyed with rhodamine is fluorescent 

 likewise. The theory of Witt thus appeared very promising 

 as an explanation of the phenomena of dyeing, but a closer 

 investigation has shown that it cannot be accepted uncondi- 

 tionally, although some modification of it may be found to 

 satisfy the experimental requirements. It has been proved 

 in a considerable number of instances now investigated 

 that the concentrations of the dye in the dye-bath and 

 in the fibre do not stand to each other in a relation 

 of simple proportionality, but the concentration in the bath 

 is roughly proportional to a power (usually 3 to 5) of the 

 concentration in the fibre. Now on the theory of solid 

 solutions this indicates that the molecule of the dye in the 

 water is three to five times as great as the molecule of the 

 dye in the silk ; but this cannot be the case, for the mole- 

 cule of the dye-stuff in aqueous solution can be shown by 

 other means to be the simplest possible. The numbers 

 rather indicate analogy to the process known as absorption 

 from solution. Substances like animal charcoal and 

 platinum black have the property of condensing gases in 

 the extensive surface they present. Similarly they can 

 abstract certain substances from solution, as may be seen in 

 the employment of animal charcoal for the decoloration of 

 solutions. The relation between the concentration in the 

 solution and that in the charcoal proves to be of the same 

 kind as is met with in dyeing, so that we are led to suspect 

 a similarity in the nature of the two processes. The so- 

 called "iodide of starch," the blue compound formed when 

 starch and iodine solution are brought into contact, would 



