GENEVA SOCIETY OF PHYSICS AND NATURAL HISTORY. 219 



ing power of those faces. Light is not the cause of the phenomenon of 

 the rotation of the wheel. This our colleague demonstrated by concen- 

 trating on the black face of the disk of an extremely sensitive radiometer 

 the light of the full moon passing over the meridian. If a large lens or 

 a concave mirror 0.55 metre (2 feet) in diameter is employed, the intense 

 light of the focus renders the paddle motionless. The phenomenon has 

 therefore a calorific origin. M. Wartmanu also reported an experiment 

 due to Dr. Schuster, according to which, when the motion of the wheel 

 is impeded, it is the envelope of the radiometer, delicately suspended, 

 that tends to turn in an opposite direction. There is therefore a mechan- 

 ical reaction. Finally, M. Wartmaun dwelt upon the fact that the degree 

 of vacuum exerts a very sensible influence on the rate of each instru- 

 ment under experiment. With the same radiometer, this vacuum does 

 not appear to preserve the same value after the energetic action of the 

 calorific sources. 



M. Th. Turrettiui submitted a communication on the different boring 

 implements employed in tunneling St. Gothard, and explained the 

 various apparatus, with working plans. The drill invented by him- 

 self, and bearing his name, is distinguished from others in two essential 

 points : 1st. The system of air-distribution, effected by the reciprocat- 

 ing motion of the piston, which is produced by an arrangement of the 

 piston and piston-rod in two separate pieces, and by the assistance 

 of a system of differential pistons; 2d. The automatic advance and 

 recoil of the machine is effected by utilizing the action of compressed 

 air upon the percussion cylinder, in order to produce the tightehing and 

 loosening of the apparatus, which permits or stops that advance by the 

 l)osition itself occupied by the j^iston-rod. This machine makes 600 

 strokes per minute. 



3. Chemistry, — Professor Mari gnac gave to the society an account of his 

 researches, published in the Archives, !N"o. 218, p. 113, on the specific heat of 

 saline solutions. The specific heat depends on the nature of the acids and 

 the bases of the salts, and also on other circumstances, which do not seem 

 connected with the greater or less tendency of salts to combine with water 

 to form definite and crystallizable hydrates. It is much less in general 

 than the sum of the specific heat of the separate elementsof the saline solu- 

 tions; but the greater part of the acetates and acetic acid dissolved in 

 water present the reverse. M. Marignac is of the opinion that the diminu- 

 tion of the specific heat of solutions is due to the disengagement of heat, 

 resulting from a dissociation of the definite hydrates contained in them. 

 This does not occur among alloys, because they cannot undergo disso- 

 ciation so long as they are below their point of fusion. 



M. Ador gave a statement of the research which he carried out in con- 

 nection with M. Albert Eilliet, on the constitution of benzine. This 

 investigation, with its numerous details, was inserted in the Archives, 

 :So. 219, p. 263. 



M. Eugene Demole made a communication on the compound C* H^ I O, 



