PHOTOCHEMISTRY 277 



The receivers are usually made of thin silver. Details of construction 

 may be found in the literature (2; 6, page 435). Copper and constantan 

 wires are easy to work with as they may be readily soldered, using a 

 flux of rosin. Large-area thermopiles (1 by 4 cm.) for photochemical 

 purposes may be conveniently made with a single receiver of thin silver 

 (0.01 mm. in thickness), to the back of which the soldered and lacquered 

 thermocouple junctions are attached with de Khotinsky cement. The 

 thermopile is connected directly to a galvanometer having a sensitivity 

 of 5 to 10 mm. per microvolt and a critical damping resistance of 10 to 

 25 ohms, and calibrated with a carbon-filament lamp from the U. S. 

 Bureau of Standards. For crude work, in the absence of such a standard 

 lamp, it may be assumed that a new 60-watt tungsten-filament lamp 

 gives radiation such that at a distance of two meters approximately 1 erg 

 falls on 1 mm.'^ of the receiving surface per sec. Details for exact cali- 

 bration (3; 6, page 437) are supplied with the lamps which may be pur- 

 chased from the Bureau of Standards at a small cost. It is necessary 

 to cover the thermopile with a window in order to prevent stray currents 

 of air. A heavy metal box surrounding the receiver helps to maintain a 

 constant temperature and prevent drift of the zero point. It is necessary 

 to make the window of quartz in order to standardize the thermopile 

 because glass windows absorb a considerable quantity of the infra-red 

 heat radiation from the standardizing lamp. Glass windows may be 

 used if it is not necessary to calibrate the thermopile with a standard 

 lamp. A quartz window makes the thermopile suitable for work in the 

 ultra-violet as well as in the visible. 



The quantity of light may be determined also by an actinometer 

 which contains a photochemically sensitive system which can be followed 

 by chemical analyses. One of the best materials for use as an actinom- 

 eter is a solution of uranyl sulfate and oxalic acid. In this solution, 

 0.6 of a molecule of oxalic acid is decomposed for each quantum of light 

 absorbed throughout the whole range of wave-lengths between 2500 



o 



and 4200 A. Variations at the different wave-lengths amount to 0.1. 

 Details may be found in the paper of Leighton and Forbes (12). The 

 decomposition of oxalic acid is readily followed by titrations with a 

 standard solution of potassium permanganate, in sulfuric acid solution. 

 Chemical Change. — The course of a photochemical process may be 

 followed in a number of different ways, either by chemical analysis or 

 by physical measurements. Whenever possible it is desirable to use a 

 physical measurement which does not affect the reacting system such, for 

 example, as pressure change of a gas, or electrical conductance, or evolu- 

 tion of gas. A very convenient method consists in using the absorption 

 of light as a means of determining the amount of unknown material. 

 In fact, the thermopile readings may be used for measuring both the 

 intensity of light absorbed and the quantity of material in solution, 



