PART 2. PHOTOCHEMICAL INVESTIGATIONS 



The principles and experimental technique of photochemistry may- 

 be learned best from the many excellent researches that are now appearing 

 in the scientific journals. A brief survey of a few of these is given in 

 the following pages. The reactions are chosen to illustrate specific 

 theories and quantitative experimentation, and no attempt is made to 

 cover the whole field of photochemistry. Complete references to earlier 

 work may be found in the references cited in the bibliography. 



Acetylene. — Acetylene gas polymerizes to a yellow solid at wave- 

 lengths below 2534 A. Light of longer wave-length is inactive. The solid, 

 called "cuprene," is similar to a resin and it is so insoluble that its molec- 

 ular weight has not yet been determined. Oxidation and analysis show 

 that the material has the formula (CH)^. Lind and Livingston (36) 

 found a quantum yield of about seven molecules polymerized for each 

 quantum absorbed in the region between 2334 and 2054 A. A slight 

 temperature coefficient was reported. A focal isolation method was used 

 in which the shorter wave-lengths were converged most by a quartz 

 lens and passed through a small opening. 



Acetone. — The photolysis of gaseous acetone has been carefully 

 investigated in several different laboratories during the last few years. 

 The chief reaction is: 



(CH3)2CO -hhv = C2H6 + CO 

 According to this reaction, the decomposition produces a doubling 

 of the pressure. Methane and sometimes hydrogen are found with 

 the ethane, but the carbon monoxide always comprises about 50 per cent 

 of the products. 



Damon and Daniels (12) sealed off acetone vapor in a rectangular 

 quartz cell and measured the pressure through a sensitive quartz dia- 

 phragm, provided with an external platinum contact which closed 

 an electrical circuit. A measured air pressure was balanced against the 

 pressure of the acetone and its decomposition products. The cell was 

 placed in a water thermostat at 56.5°, making possible the use of acetone 

 vapor at a pressure of one atmosphere. Monochromatic light from a 

 capillary lamp and large quartz monochromator was passed through 

 the cell onto a thermopile of large area, placed at the rear of the cell 

 and contained in a submarine jacket completely immersed in the thermo- 

 stat water. 



For zero readings of the thermopile and galvanometer a second quartz 

 cell, identical in dimensions with the reaction cell, was placed at tlie side 

 and shifted into the beam of the light. 



282 



