212 SCIENCE PROGRESS 



sulphite when the reaction proceeds thermally. The same 

 reaction has recently been investigated photochemically by 

 Mathews (5), the solution being exposed to the ultra-violet rays 

 from a quartz mercury lamp. 



In the first place it was discovered that uranium salts act 

 as positive catalysts for the photochemical reaction in pro- 

 portion to their concentration. Some idea of the normal rate 

 of the reaction as carried out by Mathews in the absence of 

 any catalysts will be gathered from the fact that a 0*2 normal 

 solution of sodium sulphite was oxidised completely by the 

 light in less than three hours. One very remarkable discovery 

 is that copper sulphate, which Titoff found to be a very strong 

 positive catalyst for the thermal reaction, exerts no measur- 

 able effect upon the photochemical reaction. Pyridine strongly 

 inhibited the reaction when present in the proportion of 5 cc. 

 to one litre of solution, whilst esters had a much weaker 

 (negative) effect. Five drops of benzaldehyde to a litre of 

 solution inhibited the reaction, whereas the same amount of 

 glycerine had scarcely any negative effect. Bigelow had 

 found that benzaldehyde and glycerine both acted as strong 

 negative catalysts for the thermal reaction. Other substances 

 examined, all of which exhibited a negative catalytic effect 

 were, urea (very feeble effect), phenol, quinine-sulphate, and 

 hydroquinone. In the presence of quinine sulphate and 

 pyridine the solution acquires a green colour under the influ- 

 ence of light, and with hydroquinone it becomes opalescent. 



It would appear from the above results that we have to 

 do with selective absorption of light at certain regions of the 

 spectrum. It is very necessary therefore to possess a com- 

 plete knowledge of the absorption spectra of all the substances 

 participating in the reaction. Mathews indicates that such 

 measurements in the ultra-violet region are to be undertaken. 

 It may be possible in this way to link up more closely the 

 photochemical and thermal reactivities of various substances. 



Bibliography 



1. Bigelow, Zeitsch. physikal. Chem. 26, 493, 1898. 



2. YOUKG, Journ. Amer. Chem. Soc. 23, 119, 450, 1901 ; ib. 24, 297, 1902. 



3. TlTOFF, Zeitsch. physikal. Chem. 45, 641, 1903. 



4. Luther, ibid. 45, 662, 1903. 



/Mathews and Dewey, /ourn. Physical Chem. 17, 211, 1913. 

 \ and Weeks, Journ. Amer. Chem. Soc. 39, 635, 1917- 



