1876.] On the Specific Volumes of Liquids. 283 



only four millionths of a grain is a sufficient proof that the indications 

 of this instrument, like those of the apparatus previously described by 

 the author, follow rigidly the law of inverse squares. An examination 

 of the differences between the separate observations and the mean shows 

 that the author's estimate of the sensitiveness of his balance is not exces- 

 sive, and that in practice it will safely indicate the millionth of a grain. 



One observation of the weight of sunlight is given ; it was taken on 

 December 13 ; but the sun was so obscured by thin clouds and haze that 

 it was only equal to 10*2 candles 6 inches off. Calculating from this 

 datum, it is seen that the pressure of sunshine is 2*3 tons per square 

 mile. 



The author promises further observations with this instrument, not 

 only in photometry and in the repulsion caused by radiation, but in other 

 branches of science in which the possession of a balance of such incredible 

 delicacy is likely to furnish valuable results. 



February 17, 1876. 

 Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. " Researches upon the Specific Volumes of Liquids. " By T. 

 E. Thorpe, Ph.D., F.R.S.E., Professor of Chemistry in the 

 Yorkshire College of Science, Leeds. Communicated by Prof. 

 A. W. Williamson, For. Sec. R.S. Received January 14, 

 1876. 



II. On the Specific Volumes of certain similarly constituted 

 Inorganic Chlorides. 



The results of the observations made by Pierre and Kopp upon the 

 boiling-points, specific gravities, and thermal expansibilities of the tri- 

 chlorides and tribromides of phosphorus, arsenic, and antimony have led 

 Kopp to suppose that the specific volumes of phosphorus, arsenic, and 

 antimony, in their liquid combinations, may be identical. The same con- 

 clusion has been drawn with respect to tin, titanium, and silicon from 

 Pierre's observations upon the tetrachlorides of these elements *. 



* Ann. der Chem. u. Pharm. xcvi. p. 319. In his original paper Kopp remarks that the 

 specific volume of antimony, from his observations on the chloride, SbCl 3 , and the bro- 

 mide, SbBr 3 , is decidedly larger than that of phosphorus and arsenic. But the larger 

 value is in part due to the atomic weight of antimony being taken as 129. If the more 



t2 



