( 645 ) 



layers o\' the ciTslal Uicii luH'omc dull and a lij^litcr greoii, Iho 

 iiilrösoheiizoic acid, which ci-yslalliscs oid, is now however whil(^ 

 and coiise(|ueiilly hiiiiolecnlair '); liiially the surface of the crystal 

 hecoines quite white and opaiiue. The process then aj)i)arcntly comes 

 to a standstill because the snidiuht cainMjt any longer penetrate tiie 

 interior of the crystal or only in an insufticient degree. In this case 

 the interior of a snfliciently big ci-yslal still contains a green trans- 

 [Kireiit nucleus. 



The titration of IInc diflereiit specimens has given the following 

 result : 



After 2\/.^ day about 3 "/i. ^^ nitrosobenzoic acid. 



II i<) It It ii It It 



It 'J ^ It ft J 'J It It 



II 34 ,/ // 24 ,/ I, 



Tlie surfaces of the last crystals had turned quite white. 



Conclusions as to the velocity of transformation cannot of course 

 be drawn from these ligures, as on the one hand the source of light 

 varied too much in intensity, whilst on the other hand the crystals 

 were of a different thickness. 



It Avas considered of importance to try and determine the maximum 

 soluI)ility of o-nitrosobenzoic acid in o-nitrobenzaldehyde. From the 

 surface of those green crystals, which commenced to deposit the 

 ^vhite acid, the latter was therefore as tar as possible removed by 

 mechanical means. By titration 2.6 "/„ of nitrosobenzoic acid was 

 then found ; if now we may assume that the concentration of the 

 acid inside tiie crystal is not smaller than that at the suj'face the 

 saturated solid solution contains about 2.6 mols. of acid per 100 mols. 



Another conclusion may still be drawn from the above, namely 

 that o-niti'osol)eir/aldehy(le is capal)le of forming mixed crystals with 

 2.6 mols. of o-nitrosobenzoic acid; whether these two substances 

 are isomorphous is not known as the system of crystallisation of 

 nitrosobenzoic acid has not been determined. Vei'v j)robably they 

 are not isomorphous as otherwise the power to tbrm mixed crystals 

 would occur oxer a largei- interval or even for all proportions. 



^) It has not been possible to ascertain, by the ortlinaiy moans at disposal, not 

 even by the liigliesl possible enlargement, thai the o-nitrosobenzoic acid formed 

 is crystalline. This cannot be a matter of surprise it' wc consider that the separa- 

 tion of the acid proceeds very lapidly and lliat tlie diffusion in solid solution is 

 particularly slow. Still we may speak here of crystallisation as the separated 

 sul)slance, in contrast to amorphous compounds, exliibits definite physical constants 

 (fixed melting point, solul)ilily clcj. 



44* 



