PHILOSOPHICAL SOCIETY OP WASHINGTON. 219 



nations of known phenomena, or to present old knowledge in a 

 new and more imposing dress, but to serve the higher purpose 

 of suggesting new experiments and new phenomena, and thus to 

 assist in enlarging the bounds of science, and extending the 

 power of mind over matter; and unless the hypothesis can be 

 employed in this way, however much ingenuity may have been 

 expended in its construction, it can only be considered as a scien- 

 tilic romance worse than useless, since it tends to satisfy the mind 

 with the semblance of truth, and thus to render truth itself less 

 an object of desire."* 



Light and Heat. — Henry also made important investigations 

 on some peculiar phenomena connected with light and heat. 

 For the i)urpose of experimenting on sun light he devised in 

 1840, a very simple form of heliostat, based on the suggestion 

 of Dr. Young, whereby the solar ray was received into an upper 

 room in a direction parallel to the earth's axis, requiring there- 

 fore only an equatorial movement of the reflector; which was 

 effected "by the aid of a common cheap pocket watch placed on a 

 small hinged board set by a screw to the angle of latitude. The 

 mirror mounted on a swivel and properly balanced, presented 

 no sensible resistance to the running of the watch, which was 

 arranged for the 24 hour rotation by a watch-maker of Princeton. 

 Tlie whole cost of the completed instrument (including the time- 

 movement) was but sixteen dollars. If any particular direction 

 of the ray was required, it was only necessary to place a stationary 

 mirror in the fixed path of the ray, adjusted to the desired 

 angle, f 



In 1841, on repeating experiments of Becquerel and Biot on 

 " Phosphorescence," he discovered some new characteristics in 

 the emanation (particularly when excited by electrical light) 

 which had not befpre been observed. | These were more fully 

 detailed in a communication made to the American Philosophical 

 Society, in 1843, "On Phosphorogenic Emanation.'' This phe- 

 nomenon had been first observed in the diamond, when taken into 

 a dark room immediately after exposure to direct sunlight, or 

 to a vivid electric spark ; and was afterward observed in several 

 other substances, — notably in the chloride of calcium — "Ilom- 

 berg's phosphorus. "§ It had also been shown by Becquerel that 



* Proceed. Am. Phil. Son. Nov. 6, 1846, vol. W. pp. 287-290. 



t Proceed. Am. Phil. Soc. Sept. 17, 1841, vol. ii. p. 97. 



I Proceed. Am. Phil. Soc. April 16, 1841, vol ii. p. 46. 



§ Ilomberg's phosphorus is chloride of caloium prepared by melting 

 one part of sal ammonia with two parts of slaked lime. Canton's phos- 

 phorus is sulphide of calcium formed by a mixture of three parts of 

 sifted and cahnned oyster shells, and one part of fiowerd of sulphur, ex- 

 posed for au hour to a strong heat. 



53 



