Actinometer, the name generally but improperly applied to a thermometer intended to measure the heat of the solar rays. The first so-called actinometer was made by Sir John Herschel in 1825, and consisted of a thermometer with a large bulb filled with the blue solution of the ammonia sulphate of copper, enclosed in a box with plate glass on top. When exposed to the sun's rays the expansion of the liquid indicates their intensity. The instrument is nearly identical in its results with that of Pouillet, which he calls pyrheli-ometer. Recently an ordinary mercurial thermometer enclosed in a box, and used alternately in the shade and in sunshine, was described by the Rev. Mr. Hodgkinson under the name of actinometer. - A true actinometer is an instrument to measure the actinic or chemical power of the solar rays. The first contrivance to effect this object was the darkening of a surface sensitized by chloride of silver. The difficulty here was to make a preparation which was always uniformly sensitive. Dr. John W. Draper of New York discovered the important fact that of a mixture of equal volumes of chlorine and hydrogen, the amount combining to form chlorhydric acid is directly proportional to the actinic intensity of the light and the time of exposure.

He made use of this property for the purpose of practical aetinometry; while recently Bunsen and Ros-coe have devised an actinometer based on the very same principle, and giving results of the most absolute scientific accuracy. There are, however, many other actions of this kind known in chemistry which may be more conveniently employed. A solution of chloride of gold and oxalic acid will remain clear in the dark, while gold is precipitated by exposure to actinic rays, the amount of gold being proportional to the intensity of the rays and the time of exposure. See " Philosophical Transactions," 1859, p. 879; 1852, p. 139.