ox THE ESTIMATION OF SMALL EXCESSES OF WEIGHT. 23 



but a drop of sulphuric acid added to the mixture is suffi- 

 cient to evolve the gas abundantly. Aurin sulphite has 

 the composition (C,pH,4.0j)iSOjHi + 4HiO. 



As we have already showed, aurin forms very charac- 

 teristic compounds with the acid sulphites of the alkali- 

 metals, which, in accordance with the newly established 

 formula of aurin, must now be written as follows : — 



C,,H,,03,S03KH ; 



C,,H,^03,S0jNaH ; 



C,,H,,03,S0j(NHJH. 



AVe have also found that rosolic acid, or the next higher 

 homologue of aurin, forms compounds with mineral acids 

 which crystallize well. Being, therefore, a base like aurin, 

 we think its name ought to be altered ; and as it has only 

 been obtained from rosaniliue, we propose for it the name 

 I'os aurin. 



III. On the Estimation of Small Excesses of Weight by the 

 Balance from the Time of Vibration and the angular 

 Deflection of the Beam. By J. H, Poynting, B.A., 

 B.Sc. 



Read December lotb, 1878. 



While working last year on an experiment to determine 

 the mean density of the earth by the balance, I had to 

 measure such an exceedingly small difference of weight 

 that I could not at that time estimate it by means of a rider. 



