172 Prof. Tyndall on the Absorption and 



Rock-salt immediately suggests itself as the proper substance ; 

 but to obtain plates of suitable size and transparency was ex- 

 ceedingly difficult. Indeed, had I been less efficiently seconded, 

 the obstacles thus arising might have been insuperable. To the 

 Trustees of the British Museum I am indebted for the material 

 of one good plate of salt ; to Mr. Harlin for another; while Mr. 

 Lettsom, at the instance of Mr. Darker*, brought me a piece of 

 salt from Germany from which two fair plates were taken. To 

 Lady Murchison, Sir Emerson Tennant, Sir Philip Egerton, and 

 Mr. Pattison my best thanks are also due for their friendly 

 assistance. 



The first experiments were made with a tube of tin polished 

 inside, 4 feet long and 2*4 inches in diameter, the ends of which 

 were furnished with brass appendages to receive the plates of 

 rock-salt. Each plate was pressed firmly against a flange by 

 means of a bayonet joint, being separated from the flange by a 

 suitable washer. Various descriptions of leather washers were 

 tried for this purpose and rejected. The substance finally 

 chosen was vulcanized india-rubber very lightly smeared with a 

 mixture of bees-wax and spermaceti. A T-piece was attached 

 to the tube, communicating on one side with a good air-pump, 

 and on the other with the external air, or with a vessel contain- 

 ing the proper gas. 



The tube being mounted horizontally, a Leslie's cube contain- 

 ing hot water was placed close to one of its ends, while an excel- 

 lent thermo-electric pile, connected with its galvanometer, was 

 presented to the other. The tube being exhausted, the calorific 

 rays sent through it fell upon the pile, a permanent deflection of 

 30° being the consequence. The temperature of the water was 

 in the first instance purposely so arranged as to produce this 

 deflection. 



Dry air was now admitted into the tube, while the needle of 

 the galvanometer was observed with all possible care. Even by 

 the aid of a magnifying lens I could not detect the slightest 

 change of position. Oxygen, hydrogen, and nitrogen, subjected 

 to the same test, gave the same negative result. The tempera- 

 ture of the water was subsequently lowered so as to produce a 

 deflection of 20° and 10° in succession, and then heightened till 

 the deflection amounted to 40°, 50°, 60° and 70°; but in no 

 case did the admission of air, or any of the above gases into the 

 exhausted tube, produce any sensible change in the position of 

 the needle. 



It is a well-known peculiarity of the galvanometer, that its 

 higher and lower degrees represent different amounts of calorific 



* During the course of the inquiry, I have often had occasion to avail 

 myself of the assistance of this excellent mechanician. 



