170 ESSAYS BIOGRAPHICAL AND CHEMICAL 



Again, interesting experiments have been made by 

 Landolt as regards constancy of weight. Having sealed 

 up in an inverted U-tube two substances capable of acting 

 on each other, such as silver nitrate and sodium chloride, 

 each substance in solution occupying one limb of the 

 tube, he weighed the tube with the utmost accuracy ; the 

 possible error might be one part in a million. On inverting 

 the tube, the two solutions mixed, and the reaction took 

 place. It was again weighed. For long, Landolt supposed 

 that he had detected small changes in weight, sometimes 

 negative, sometimes positive; but he was able to trace 

 these changes to the porous nature of glass. On employ- 

 ing tubes made of fused quartz, no change of weight could 

 be detected after the reaction was over. Apparently, 

 therefore, no change of weight takes place as the result of 

 a chemical reaction, provided nothing leaves or enters the 

 vessel in which the reaction goes on. 



A very ingenious experiment of Joly's deserves mention. 

 It was designed to try whether any change of mass occurs 

 on mixing two reacting bodies, and the disposition of the 

 apparatus was somewhat like that devised by Landolt. 

 But instead of utilising the attraction of the earth in order 

 to estimate whether the mass had changed or not, the 

 inertia of the substances and of their mixture was deter- 

 mined. The vessel containing the substances to be mixed 

 was suspended to the arm of a torsion-balance, the arm of 

 which was at right angles to the direction of motion of the 

 earth, which is known to be at the rate of about 30 miles 

 a second through space. If matter had been created 

 during the chemical change, then the created matter 

 would not partake of the earth's velocity, and a retarda- 

 tion, made manifest by the rotation of the arms of the 

 torsion-balance in one direction, would have been observed ; 

 and. if, on the other hand, matter had been destroyed, an 

 acceleration would have shown itself. The experiments 



