146 Royal Society : — 



ride of sodium. The following is a comparison of the theoretical 

 and experimental results : — 



EXPERIMENT. THEORY. 



Iodine 3-69 3-81 



Bromine 3-51 3-34 



Chlorine 2-92 3-02 



I have availed myself of this method in analysing several silver 

 ores containing chloride, bromide and iodide of silver found in Chili, 

 the formulae of which I subjoin ; — 



Chloride of silver Ag CI. 



Chlorobromide of silver 2Ag CI, Ag Br. 



Chlorobromide of silver 3Ag CI, 2Ag Br. 



Chlorobromide of silver Ag CI, 3Ag Br. 



Bromide of silver Ag Br. 



Iodide of silver Ag I. 



"Note on the Density of Ozone." By Thomas Andrews, M.D., 

 F.R.S., Vice-President of Queen's College, Belfast, and P. G. Tait, 

 M.A., Fellow of St. Peter's College, Cambridge, and Professor of 

 Mathematics in Queen's College, Belfast. 



It is known that ozone can only be obtained mixed with a large 

 excess of oxygen. In a former communication by one of the authors 

 of this note, it was shown that in the electrolysis of a mixture of 8 

 parts of water and 1 of sulphuric acid, the mean quantity of ozone 

 does not exceed "0041 gramme in a litre of oxygen, or xto*^^ part. 

 By using a mixture of equal volumes of acid and water, the relative 

 quantity of ozone may be doubled ; but even with the ozone in this 

 more concentrated state, the ordinary methods of determining the 

 density of a gas are plainly inapplicable. The difficulty of the pro- 

 blem was farther increased by the rapid action of ozone on mercury, 

 which rendered it impossible to collect or measure the gas over that 

 metal ; and the tension of aqueous vapour, as well as the gradual 

 destruction of ozone by water, prevented the use of the latter. After 

 numerous trials, the method finally employed was to measure the 

 change of volume which occurs in exposing a gaseous mixture con- 

 taining ozone to a temperature of 230° C, or upwards. The volume 

 of the gas after this treatment was invariably found to have increased ; 

 and by eliminating the effects of altei-ation of temperature and pres- 

 sure during the course of the experiment, by the aid of a similar vessel 

 to that containing the ozone, the authors succeeded in estimating the 

 change of volume which took place, to an extremely small fraction 

 of the entire amount. 



The vessels employed in different experiments varied in capacity 

 from 200 CC. to GOO CC, and terminated in tubes of about 2 milli- 

 metres in diameter, bent in a U-form and containing sulphuric acid. 

 The amount of ozone was ascertained by passing a stream of the gas 

 through two other vessels, one placed on each side of the vessel to 

 be heated, and afterwards analysing their contents by the method 



