ON ORGANIC COMPOUNDS 85 



concentration in the cell is kept at a low level. If it be sought to 

 imitate this in a solution of formaldehyde, the concentration must 

 be increased so that the decrease of osmotic pressure may yield 

 energy to supply that required in conjugation, or assist energy supply 

 from without, such as light energy, to increase the potential towards 

 chemical union. 



This is what has been done in the experiments here recorded, in 

 which a reducing substance has been obtained by subjecting com- 

 paratively concentrated formaldehyde solutions to the light of the 

 quartz mercury- vapour lamp. 



When six molecules of formaldehyde condense to form one 

 molecule of a hexose, there is only one molecule of dissolved material 

 contributing to keep up the osmotic pressure where there were 

 formerly six, and a corresponding amount of osmotic energy has 

 disappeared as such, and been utilised to yield the slightly higher 

 content of chemical energy which the hexose possesses over that of 

 the formaldehyde which went to form it. The energy so yielded 

 will evidently be proportional to the osmotic pressure at which the 

 formaldehyde molecules disappear that is to say, to the concentra- 

 tion of the formaldehyde solution. The osmotic pressure represents 

 the intensity or potential factor of the osmotic energy, and this comes 

 into equilibrium with the intensity factor of the chemical energy, 

 tending to disrupt hexose into formaldehyde and set energy free. 



This osmotic energy supply is sufficient to yield the amount 

 required, and hence, in this type of reaction, the light plays mainly 

 the part of a catalyst, and not a provider of energy as in the synthesis 

 of formaldehyde from carbon dioxide and water, or in the synthesis 

 of nitrites from nitrates. 



Hence it arises that very dilute solutions of formaldehyde do not 

 yield hexoses under the action of light, and that dilute solutions of 

 sugars do not condense to form disaccharides. Conversely, if dilute 

 solutions of the higher condensations, such as disaccharides or 

 polysaccharides, be exposed to light, they split up into simple 

 sugars, and these invariably yield formaldehyde, as will be shown 

 later in this chapter. 



The first synthetic sugars were obtained by Butlerow, 1 by Loew, 



1 Butlerow, Liebig's Ann., Bd. cxx., p. 295; O. Loe\\ r ,Berichte d. D. Chem. 

 Gesellsch., Bd. xix., p. 141 (1886); Bd. xx.,pp. 141 and 3039 (1887); Bd. xxi., 

 p. 278 (1888); Fischer and Passmore, Berichte d. D. Chem. Gesettsch., Bd. xxii., 

 p. 359 (1889). 



