of Selenium and Sulphur to Light. 185 



whether the substances are heated together before or after the 

 formation of the cell. Sulphur containing sufficient metallic 

 sulphide to render it a conductor of electricity might be used 

 in the construction of a cell which might be expected to be 

 sensitive to light without any preliminary annealing. This 

 turned out to be actually the case. 



Silver was the metal chosen for the experiments on account 

 of the facility with which it combines with sulphur. 



Cell JVo. 1. — Five parts of sublimed sulphur and one part 

 of precipitated silver were heated together in a porcelain cru- 

 cible for about two hours. The mixture was from time to 

 time stirred with a glass rod and w r as finally allowed to settle, 

 so that the bulk of the sulphide and any free silver wdiich might 

 remain fell to the bottom of the crucible. When the tem- 

 perature was slightly above the melting-point the liquid 

 sulphur, which was perfectly mobile, though black with 

 minute suspended particles of sulphide, was poured off for 

 use. Two wires of fine silver * w T ere then coiled side by side 

 around a strip of mica 50 millim. long and 27 millim. wide ; the 

 wires were about 1 millim. apart, and care was taken that they 

 did not touch each other at any point. Some of the melted 

 sulphur was spread evenly over one surface of the mica, the 

 two wires being thus connected with each other through half 

 their entire length by a thin layer of the prepared sulphur. 

 When cold, this cell was connected in circuit w 7 ith a battery 

 and a galvanometer. It was found to conduct electricity, 

 but its resistance was very high, being probably between 

 20 and 30 megohms. With the object of partially bridging 

 over the intervals between the wires, the sulphur w r as melted 

 by laying the cell upon a hot plate, and a piece of very thin 

 silver-foil, measuring 25 millim. by 10 millim., was laid upon 

 its surface: this was probably entirely converted into sulphide 

 before the cell was again cold. The cell was now found by 

 a bridge-measurement to have a resistance of 900,000 ohms f. 

 Once more it w<as connected with a Leclanche cell and a 

 suitably shunted galvanometer; the deflection was noted, 

 and a piece of magnesium wire was burnt at a short distance 

 from the sulphur. The deflection was immediately more than 

 doubled; and w T hen the magnesium w r as extinguished, the spot 

 of light at once returned to very nearly its original position. 

 The effect was almost as great when a glass trough containing 

 a saturated solution of alum was interposed between the 

 sulphur and the burning magnesium J. 



* Supplied by Messrs. Johnson and Matthey. 



t This resistance was afterwards found to be very variable, and it was 

 never the same with a direct and a reverse current. 



% This cell was exhibited in action at the Meeting of the Physical 

 Society on May 23rd, and at the Soire'e of the Royal Society on June 10. 



