AC'TINOM'ETER (Gk. krie, oktis, ray + larpor,a/ctron.measure). An instrument for meas uring the effect of the sun's rays in producing chemical, i.e., actinic streets. As originally devised by Sir .John Herschel, this title was applied by him to a thermometer whose bulb was filled with a blue solution of ammonia and sulphate of copper; the expansion of this solu tion by absorbing, the sun's rays was supposed to measure the quantity of blue light or chemical rays in the beam of sunshine. At the present time it is known that actinometers, properly so called, measure only the effects of the energy transmitted to us in specific portions of the solar spectrum. in sonie arrangements this energy is all turned into heat and measured by its expansion effect. In other forms of apparatus it does molecular work of a chemical nature and is measured by these effects, as when a mixture of chlorine and hydrogen is converted into hydro chloric acid and the quantity of acid that is formed in years of time is the measure of the intensity. This includes the basis of the methods
of Draper and Bunsen and Roscoe. NVhen a mixture of ferric-oxalate and chloride of iron dissolved in water is exposed to sunshine it gives out carbonic acid gas: this is the basis of Marchand's apparatus. A photographic plate exposed for a short time receives an impression whose intensity may be measured on a scale of tints or shades and made the basis of a deter mination of the intensity of the sunshine. This method has been worked out by Bigelow and others. In general any apparatus for measuring the chemical effects of radiation from any source constitutes an actinomete• properly so called, but the name is often improperly applied to apparatus that measures the total heating effect, as was the case in Herschel's apparatus; it is even now applied to the Arago-Davy and the Chwolson apparatus, all of which are, prop erly speaking, forms of pyrheliometer, and will be found described under that head.