272 



Dr. W. M. Bayliss. 



[Apr. 19, 



At the end of the above experiment, the solution was pipetted out of the 

 osmometer and its concentration determined by evaporating to dryness a 

 known volume and drying the residue to constant weight in a toluene 

 bath in the usual way. It was found to be - 30 per cent., so that, if the 

 dye had been present as separate molecules, the osmotic pressure should 

 have been 0"30/1 x 27l"4 = 81*4 mm. Hg. The actual value found was 

 79 - 3 mm. Hg, or 97 per cent. 



In other experiments the agreement with theory was not so good, e.g. 

 207 mm., instead of a theoretical 228 mm., or 91 per cent., for a - 84-per- 

 cent. solution at 30 Ol 2 ; 77'4 instead of 84, or 92 per cent., for a 0-309-per- 

 cent, solution at 30 o- 7 ; and 128 instead of 146, or 88 per cent., for 

 0"58-per-cent. solution at 10°. 



It is obvious that these values could only be obtained if the greater part 

 of the elements responsible for the production of osmotic pressure were 

 present as single molecules, since any value greater than one-half the 

 theoretical implies that a part of the active elements consists of single 

 molecules. 



When the molecular weight of haemoglobin is calculated from the content 

 in iron, a value of about 12,000 to 14,000 is obtained.* Now this is the 

 same number obtained from the osmotic pressure determinations of Hufner 

 and Gansser,f and of Eoaf.J Haemoglobin, therefore, exists in solution in 

 single molecules, although, like congo-red, it does not pass through parch- 

 ment-paper. The molecular weight of congo-red, however, is very much less 

 than that of haemoglobin, only about one-twentieth in fact, so that it is more 

 surprising to find it to behave as a colloid. 



On this account it is advisable to examine how far congo-red exhibits 

 other properties associated with colloids. To what degree does a molecule 

 of such dimensions show the characteristics of matter in mass, possessing 

 surfaces ? 



In the first place, what appearance does a solution of congo-red show in 

 the ultra-microscope ? According to Michaelis§ the particles present are 

 sub-microscopic, that is, resolvable into separate bright points. The same 

 statement is made by Pelet-Jolivet and "Wild.il My observations are not 

 entirely in agreement with those of the investigators mentioned. There are 

 undoubtedly, a few scattered bright points to be seen, but these only 



* See Schultz, ' Die Grosse des Eiweissmolekuls,' p. 31, Jena, 1903. 

 t Loc. cit. 

 % Loc. cit. 



§ ' Deutsche Med. Wochensch.,' No. 42, 1904. 

 || Loc. cit. 



