36 MR. H. C. SORBY OK THE COLOURING-MATTERS OF 



1 



water often change rapidly, owing to the development of various 

 minute organisms ; but if clean white lump sugar be added to the 

 solution until no more is dissolved at the ordinary temperature, 

 decomposition occurs very slowly, and does not appear to occur 

 at all in hermetically sealed tubes. Having, then, prepared a so- 

 lution, its compound nature may be proved by various methods ; 

 but that to which I am the most anxious to direct attention de- 

 pends on the fact that some of the colouring-matters are decom- 

 posed when heated to the temperature at which albumen coagu- 

 lates, whereas others resist one considerably higher. Thus, in 

 the case of the coloured solution in syrup obtained from Oseilfa* 

 toria by carefully keeping it in a water-bath at a temperature of 

 75° C, the pink phycocyan is soon changed into an insoluble pink 

 substance ; and as soon as the change is complete and the absorp- 

 tion-band at the yellow end of the green has disappeared, on filter- 

 ing the liquid a clear solution of purple phycocyan is obtained. 

 By comparing, side by side, the spectra of the original and of the 

 cold but previously heated solution, we can see very clearly that 

 the bands of the purple phycocyan remains nearly as at first, 

 whilst little or no trace of the other can be detected. These facts 

 will be better understood by means of the following figure. 



700 



400 



I 



II. 



Fig. 1. Spectra of the colouring-matters from Oscillatorin. I. In natural state. 



n. Heated to 75° C. 



This and fig. 2 are both drawn in proportion to wave-lengths ; 

 and therefore the red end appears broader, and the blue narrower 

 than when seen in an ordinary spectroscope. 



In a similar manner we can prove that the beautiful purple 

 solution obtained from Porphyra vulgaris is due to a mixture of four 

 different substances. In a natural condition it gives a spectrum 

 with four well-marked bands and one much fainter; and on heating 

 the aqueous solution to 65° C, the two at the red end disappear 

 and the other three remain. If the natural solution saturated with 





