428 COLLOIDS IN BIOLOGY AND MEDICINE 



and 0), gelatinized and then placed in running water until every 

 trace of acid was removed. These strips were then placed in 0.5 

 per cent dye solutions for 10 minutes and then washed in running 

 water until the wash water was almost entirely colorless. The re- 

 sults are shown in the table on page 429. 



It follows from these experiments that the staining is more intense, 

 the denser the stained substance, in the case of easily diffusible 

 dyes, as aurantia, methylene blue and crystal violet. Conversely, 

 in those difficultly diffusible, as chrome violet and benzopurpurin, the 

 intensity of the staining diminishes with the density. Obviously, the 

 particles of the dye are largely colloidally distributed and too large 

 to penetrate the pores of the filter. In between there are substances 

 of medium particle size in which the staining of the different samples 

 does not vary much one way or the other, as, e.g., in the case of 

 alizarin, janus red, bismarck brown, etc. An important factor in 

 these experiments is the time element, as may be seen in the first 

 column (8 per cent). The slower a dye diffuses the longer the time 

 that must elapse for it to penetrate a dense tissue and the slower the 

 color constituents that are not firmly bound by the fibers will diffuse 

 away. 



In this connection experiments of E. KNOEVENAGEL* and of O. 

 EBERSTADT* are of interest; they tested samples of acetyl cellulose 

 swollen to various degrees, for their capacity to take up methylene- 

 blue solution of 0.05 per cent. They found that the speed of ad- 

 sorption was approximately proportionate to the swelling, so that 

 dyes which penetrated greatly swollen acetyl cellulose in a few min- 

 utes required months in the case of acetyl cellulose that was not 

 swollen. 



Elsewhere we have discussed whether there are not other factors 

 involved besides the speed of diffusion and the size of the particles. 



The methods of analysis proposed by me, which have not as yet 

 been applied to organized tissues, promise fewer results in micro- 

 scopic preparations and microtome sections since the surfaces to be 

 penetrated are so thin that sufficient differences are not noticeable. 

 However, we might expect new information in the case of coarse 

 pieces of tissue which are to be examined after they are sectioned. 



In what precedes, the discussion of the dyeing process has been 

 studied only from the standpoint of the chemist and the physico- 

 chemist. Somewhat independently of it and with other means the 

 same discussion will be carried into biology. 



The dye chemist deals with the coloring of a few fibers of almost 

 constantly the same constitution, with silk and wool, which dye 

 easily with most dyes, and with cotton and related fibers which are 



