618 REPORT— 1894. 



extent supplied the deficiency, and in the case of trichloracetic acid and ortho- 

 pitrobenzoic acid I have experimentally verified the validity of the dilution law 



(l_^\y ^ *■ ^° account of these investigations will he given in the ' Journal 



of the Chemical Society ' and in the ' Zeitsch. f, physik. Chemie.' 



7. On the Effect of Dilution upon the Colours of Salt Solutions and the 

 Measurement of this Effect. By Wyatt W. Randall, Fh.D. 



The writer called attention to the conclusions deduced by Ostwald from the 

 hypothesis of electrolytic dissociation with regard to the source of the colour of 

 salt solutions, and to the work of Knoblauch, Kriiss, Traube, Arrhenius, Magna- 

 nini, Ostwald, Wagner, and others upon this subject. The experiments "and 

 conclusions of Vernon were then more particularly discussed, to show the 

 inaccuracy which the author believes characterised them. A drawing was then 

 shown of an apparatus, through the use of which the author hopes to be able to 

 determine with comparative accuracy the effect of dilution upon the colours of 

 salt solution. Since, according to the theory of electrolytic dissociation, the 

 colour of a salt solution is due to the presence in it of free coloured ions, it follows 

 that with increased dissociation the colour must become proportionally more 

 intense. One of the simplest ways to produce more complete dissociation is by 

 dilution. Hence a dilute solution, ceteris paribus, should, in proportion to the 

 amount of salt contained, show a more intense colour than a concentrated one. 

 In the apparatus shown the author examines the relative length of column of 

 coloured solutions of different concentrations which show the same intensity of 

 colour, in order to determine whether or not the colour remains proportional to 

 the concentration. Several advantages claimed for the apparatus were pointed 

 put. It is, for example, so arranged that the light transmitted through the 

 stronger solution also passes through a column of water equal in length to the 

 difference between the lengths of column of the concentrated and the dilute 

 solutions. By this means any error due to the colour, &c., of the water of the 

 solutions is presumably eliminated. The tubes through which the light is trans- 

 mitted are silver-plated, and the measurements are made in a dark room, in order 

 that all error due to diffused light may be removed. 



The results, so far as they have gone, indicate that the colour remains practi- 

 cally proportional to the concentration, whereas in the solutions examined the 

 dissociation varied in amount from about 25 per cent, to nearly 60 per cent. The 

 author, however, desires his results thus far to be considered as merely preliminary. 

 The behaviour of one solution examined suggests that the effect produced upon 

 the colour by varying the rate of dilution, which Vernon claims to have noticed 

 in the case of certain compounds of chromium, may be much more general than 

 that investigator imagined. This point will in the future receive special 

 attention. 



8. On the Distinction between Mixtures and Compounds, 

 By P. J. Hartog, B.Sc. 



The distinction between mixtures and compounds, as it is now understood, 

 dates from the controversy between Berthollet and Proust at the beginning of the 

 century. Most text-books state that Proust showed that ' the same compound 

 always contains the same elements united in the same proportions,' and imply that 

 the statement does not hold good for ' the same mixture.' Interpreted literally, 

 the statement is a mere truism, and applies equally to both classes of substances. By 

 altering its form somewhat we are led to the postulate : ' Substances in other 

 respects 'alike posse.«s the same quantitative composition '—a postulate daily made 

 use of in the laboratory, though it is not to be found in the text-books. This 

 postulate was tacitly accepted by Berthollet as well as Proust, and has nothing to 

 do with the distinction sought for ; we see, therefore, that the points at issue 



